They serve several purposes.
1) Stops big things that can jamb or throw the belt
2) stops you putting your hands / foot on the belt and having them jambed in the pulley ( for those who like to emove safety devices )
3) reduce the chances of a deck fire
One question though. How the heck would they reduce chance of a deck fire? Don't understand that one. The reason I consider removing them is because I read somewhere, where a guy had a belt jumping off due to crap getting built up in there and pushing the belt off. I think he needed to clean it out more often to prevent that, but none the less, it caused a problem. Holding grass, dirt and debris in there around your pulley is what I don't like. That would hold moisture in there risking early rust. Also, I like the idea of it being open for ease of keeping it clean, and for constant monitoring for any issues before they become issues. Sure, safety has to be thought about even more so with exposed spinning parts, so you can't let your guard down. But, you shouldn't let your guard down regardless. I do think they are put on there, and should be put on there by the manufacture to prevent liabilities for lack of paying attention of the operator. Anytime my mower or any equipment is running, my alertness is on point (or at least it should be). Not that I have never let my guard down, but the times I have, generally ended up in something breaking, or someone getting hurt (usually me). I understand the reason for them. But I feel there is more benefit with them not on there as long as you are aware of the dangers that could arise due to not paying attention. But the fire thing, I don't get.