You don't need one, but if you're breeding more than one Pokemon, then it's a great breeding tool to have. Otherwise you'd need to transfer the perfect IVs to bridge Pokemon between Egg Groups, which is unnecessary breeding if you have the perfect Ditto.
Perfect Ditto can be created via two methods:
1. Catch it (extremely tedious).
2. Use cheats to generate one (much easier).
If you're one of the few players that can generate infinite perfect Dittos, then good for you. In not, then the easiest way to obtain perfect Ditto is done without creating perfect Ditto. That's right, trade for it.
Use basic trade language to let people know that you're looking for perfect Ditto. You can likely trade rare legendaries, shiny Pokemon, Master Ball, etc. for a perfect Ditto. There are perfect Dittos in circulation (the reason being those infinite Ditto generators I was talking about before). You'll just need to search for awhile....
EDIT: You can also look online at forums or this website itself! There might be someone with a spare perfect Ditto on this site. So if you're that person please comment below.