If you don't have the right arrow spine, have a bad release, tend to draw short, don't have your broadhead on straight, etc. etc. etc. a bigger feather will straighten out your arrow faster. Down side is they are noisier, will slow your arrow down a tad, and can cost more.

I have a bunch of 5.5's I got at a bargain--I chop them down to 5" when I get ready to use them.

Chad