One day Perseus will go on to be the first of Greece's great heroes. He'll chop off Medusa' s head, slay a sea monster, rescue Princess Andromeda, and found the great city of Mycenae. In this myth, however, he just kind of cries and poops a lot. (Hey, he's a baby. That's what babies are supposed to do.)
Basically, what this myth does is give a miraculous hero a suitably miraculous birth. It just wouldn't do for someone of Perseus' stature to be conceived in a normal way, so a shower of gold it is. Also, this myth anoints Perseus with the status of demigod. Being the son of Zeus, himself, Perseus is just a little bit more awesome than the rest of us puny mortals. Although he is destined to die, he's also destined for greatness.