It is more about safety and keeping your work stable, especially when doing jobs like routing the profile on the ends of the rails on cabinet doors. The narrower the gap, the less the bit is exposed and also the quicker your piece is supported when it gets to the outfeed fence. I've made quick and easy fences with wide gaps around the bit and being able to close the gap seems to consistently make a better job.
To me, minimizing the bit exposure is like having just enough table saw blade exposed to make a rip cut. I never raise my blade more than 1/4" to 1/2" higher than needed. It is just one of the things you can do to help manage risk.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.