If you have to ask, you quite likely don't need 'em. But it's just really irritating to me to get sprayed by water from my tires. One alternative is to ride very slowly when it's wet or when you go through a puddle. One alternative is just don't ride when it's wet. Or go for the skunk-stripe look. Or get fenders.
I was out riding a while back and came to some standing water on the paved bike trail. Just then, a couple of kids maybe 11 or 12 or so came zipping up to the puddle on their 20" bikes. When they got to the puddle, they stood on their seats and coasted through. Worked for them, I guess, I'd probably hurt myself if I tried that.