It's good for keeping spammers out of forums. They eventually get around it but it takes awhile. It doesn't get rid of them entirely but it drastically cuts down on their numbers. Overall it's quite useful.
CAPTCHAs is for human verification only. I dont mint always answering captchas. I think its for their security. I read in other forum that some people is earning money from using captcha. I just dont know how.