Like @Facade, I’ll agree it’s for the same reason that women have what is essentially a tiny little penis analog – but it’s for more than just pleasure. It’s there (and nipples too) because our bodies are built on the same basic model. You can change it a bit with hormones, to give men more muscle or women more subcutaneous fat, but it’s made from the same blueprint – two legs, two eyes, stomach, gall bladder, and, yes, nipples and clits.