The chainring guard keeps pants from getting dirty and torn by rubbing against the chainring and chain. I prefer to just use a velcro strap around my pant leg if I'm riding somewhere in plain clothes. I think the strap is more reliable, but possibly less convenient. It's a good solution for someone who rides in cycling kit most of the time, but uses street clothes occasionally.

A chainring guard is more useful for someone who rides in street clothes most of the time and doesn't want to bother with a strap. But it doesn't absolve you of the need for keeping your chainring, chain, and the chain guard clean -- your pants can still get grimy riding even with a guard in place. ...so you might just end up using a strap anyway.