yes. does several things
1 keeps your chain on
2. keeps thing like rocks getting between your chain and sprocket
3. Saves your sprocket from rolling over big rocks, lead pucks, broken fiberglass pieces etc both on and off the track
I know they can be a pain sometimes when they bend or changing your sprockets. they've been around too long to be just a fad.
I was frustrated with the chain guard and tight/loose(light sprocket carrier was bent) and it was tough to tell what was wrong so i started only using Half the guard. We started to toss chains every time my son would spin or hit someone with the frontend or the rear. After 4-5 dnf's from chains and my wife giving me an earful I finally put the other side back on and my problems went away.
^^^^^ this is a typical scenario you can give it up anytime as soon as you fail to make it to the end of the race you start to realize why it was there in the first place. its a piece of mind ,better safe than sorry kinda thing . could you imagine leading a big money race or a big national and DNF because you thought you were smarter than everyone else? all the top drivers use them if that makes any difference to you.