Sourdough bread is bread that has been all or partially risen with the help of wild yeast. It is made sour because of wild lactobacilli that are picked up with the wild yeast. Where the yeast eat sugar and produce CO2 for rising- the bacteria create acid and other compounds that make the bread sour and distinctly flavored.
Because of the wild nature of the microbes, the bread will taste a little different in every place that it is made. The lactobacillus that gives San Fransisco sourdough bread its characteristic flavor is actually named Lactobacillus sanfranciscensis.
The starter can be a hassle to make and to keep. It requires almost daily attention to feed it and can waste a fair amount of flour unless you bake bread frequently. You can freeze the starter but it will require a few days of feeding before it can be used again. Many baked recipes besides bread are also delicious with sourdough starter. Sourdough waffles or coffeecake are fantastic.
Whether it is worth the effort is a personal decision. Many people don't like the flavor and you would have to love the flavor enough to make almost every bread with that flavor to justify the starter upkeep. Personally- I used to keep a starter but my kids don't like the flavor and I got tired of all my bread tasting the same. I just buy a loaf for the 2-3 times a year that I am craving a Reuben sandwich.
There are lots of styles of bread, which of course different people have different preferences for, and which lend themselves to different applications.
First, what do the holes indicate? The holes are pockets or cells in the gluten network in the loaf--so the stronger the gluten network, the larger the holes can expand during proofing. similarly, the more active the yeast, or the longer the proof, the larger the holes will have the potential to expand.
So large holes tend to be found in breads with strong gluten networks, which have longer proofing times. These loaves will have chewier texture, and yeastier flavor, which can be very delicious.
For other applications, such as a typical US-style sandwich bread, the desire is for a more tender product, which is indicative of less gluten development; also, large holes would let spreads or condiments soak through the bread more easily, which is not so desirable in a sandwich to be eaten out of hand. So these loaves are typically ones with smaller holes.
So the holes are either a factor themselves (as in sandwich bread), or indicative of other qualities of the loaf (as in so-called artisan style bread)--but either way, they are a very visible indicator of the nature of the loaf, and so getting the desired holes will help get the desired loaf qualities.
I am sure there are many more qualified bread experts who will give you a more detailed answer...
Best Answer
There are several factors that make bread be "holey". First of all we must understand that those big holes are created by "balloons" of gluten filled with CO2 and alcohol made by yeasts. Those balloons can grow in 2 ways
You would also get some bubbles with rye bread and bacteria, but the holes would be smaller. So I'll focus on wheat flour and yeasts.
So, some tips to maximize the size of the holes:
Please, note that those points are not independent. Thanks @rumtscho for pointing it out.
Don't manipulate the dough too much.
Most breads have 2 fermentations:
bulkone andproofing. Do just one. The more you handle the dough, the higher chances to degas it by breaking bubbles and letting the gas get out of the dough.High hydration
With more water the dough will be less stiff, so bubbles will be able to extend more. Also, it will be easier for the yeasts to "find" their food: sugar, so they'll produce more CO2 and alcohol.
Some flours absorb more water than others. Using a flour with high absorption index might allow you
Yeast + time
The more yeasts, the more gas will be made. But be aware not to add too much, or they'll run out of glucose. It's better to give them time to do their work.
Use strong wheat flour
The more time dough is waiting to be risen, the more gluten will be destroyed by enzymes. So using flour with a lot of gluten (strong, high % proteins, a W value over 270) will help assuring a minimum of gluten will still exist after long fermentations.
Oven spring
Yeasts continue producing CO2 until they die at 60C/140F. Also, gasses expand with heat, so it will also help holes to grow a bit (if I remember well, up to 30%). But that grown will stop when dough gets baked and strengthens, and when crust begins to form. To retard this 2 tricks are used:
Use steam in the oven the fist 1/3 or 1/4 of baking time.
Steam will keep the "outer skin" of the bread humid, so it will prevent it from getting dry and forming the crust.
Score the bread
Bread is slashed (those cuts made in its surface) to allow it to grow more easily. So inner bubbles will be able to grow more in the oven.
To allow heat enter in the dough from the bottom to up, bakers use stones in their ovens.
Develop the gluten completely
Kneading is very important, so the gluten network will perfectly developed to retain all the CO2 and alcohol.
To get high hydration doughs, bakers make a normal hydration dough (approx. 66%) with all the flour, and then add the rest of water and knead a bit more to let it absorb it.
An example of a type of bread with big holes can be seen in this Spanish forum (sorry: it's a Spanish bread and I couldn't find it in English).
Ciabattas are also a good example of big holes bread.
Some tips to minimize the holes:
Low hydration
As opposite to high hydration to get big alveolus. As low as 50%
Degas the dough
Some artisan or home bakers press the dough with their hand to avoid having too big bubbles. For certain types of bread, professional bakers use rolls to make a thin one. It's called refinement, and might be made several times (maybe 5 during proofing).
Weak flour.
With less gluten, the bubbles will break letting the gas go out of the dough.
Underproof
Use a short fermentation time so yeasts won't be able to make too much CO2.
Pierce the dough
So any excess of CO2 will leave through the holes, instead of getting kept inside the dough.
Also, to lower the production rate of the yeasts, you can also add a lot (really a lot) of salt, or a lot of sugar, or grease/oil.
As an example, a couple of breads (again in Spanish): Pan Candeal and Bollo Sevillano.