Do to help the avocados ripen

Depends on if you'll buy for instant consumption or for storage.

If you want to eat/prepare them right away, you want an avocado that feels soft on the inside, when pressing them they will cede and have that soft feeling ripe avocado has. It shouldn't cede very much though, as those are past their prime. The very good ones even smell in a nice avocadoey way a bit (or maybe that's just my insanity.)

If you want to prepare them later, you want firm ones that cede only a tiny little bit (the rock hard ones mostly never ripen in my experience,) you then store them in a porous bag (the typical brown paper bag or a newspaper) to let them ripe. To speed up the ripening, you can add an apple to the bag.

See also this related question

Yeah, I would agree with the people in the comments, and make the assumption that it had to do with concentrating the ethylene gas from the bananas.

After a bit of googling, it looks like the rice bag trick might be popular because it apparently results in a nearly ideal, stable environment for ripening.

I imagine that the rice would keep the moisture level at a lower point, the bag would contain the gas, and the mass of the rice would decrease the temperature variants (assuming that is conducive to ripening)

I found an informative article which mentioned rice bags here: http://monsterguide.net/how-to-ripen-bananas-quickly