Mostly just to get the beer carbonated, or at least mostly carbonated, but it can supress esters slightly if you push your fermentation temps to the top of a yeasts happy range. I also use it to purge kegs. I have a couple of kegs attached, one filled with sanitiser and the other empty with a regulator on the output, the co2 pushes the sanitiser through into the second leg leaving one sanktised, purged and at pressure ready to hook up for a sealed gravity transfer.

So I use a heck of a lot less co2 than I used to pushing liquid around and purging kegs.