Which purpose do the checkmark Candies serve

I wrote a whole guide you can check out, but I'll summarize here:

• As others have mentioned, comboing your special candy can be quite powerful. Each combination of two special candies, when matched, will produce a devastating effect on the board as a whole. This is somewhat situational, though, since you're not always going to have the luxury of having a bunch of special candy that is easy to combo together. However, getting your 4's, 5's, and 6's matched together is almost always a priority, since they create effects that can hit a wide swath of the board.
• Work from the bottom up. The bottom candies tend to get stuck there and stay there, and more often than not you need fresh candy down there with lots of matches in order to clear the board. Additionally, moving candies at the bottom tends to create "free" matches at the top that boost your score and keep the candies moving.
• The hint system is terrible, so avoid following its instructions. It will often tell you to match 3 and break up a match of 4/5.
• You can repeatedly enter and exit levels to re-roll the starting candies. (At least on the Android version, this does not cost you a life, unless you make a move first or the level has a timer) This is especially important where you need to clear the bottom quickly - you can reroll until you've got a 5 match or a couple of 4's, or even several 3's along the bottom.

That said, the game is very random. The whole point appears to be to make the game just hard enough that you get near the end but frequently fail. The way the company makes money is when you lose and either buy new lives or attract new players who will hopefully spend money. Therefore, they've got a vested interest in putting you in unwinnable (but not discouragingly so) situations.

I've rolled several of the levels 20-30 times in service of finding easier starting positions, and I can assure you that the starting candies are quite random. The objectives tend to be in the same positions, or at least have a fixed spawning pattern in the case of "ingredient" levels, however.

Basically make combos and assume that sheer luck will take care of the bombs when exploding combos.

Also take the advantage of the fact, that after your last move you have Sugar Rush regardless if you've reached the minimum required. This means that all unexploded combos will explode. Take advantage of that and create as many as you can without bothering to explode them.