DUMB QUESTION: What is the advantage of having/using WHMCS?

Okay Yall I have a rather moronic question. Im a newbie, Im
allowed! I understand what WHMCS does okay? Automated Billing and
all that. But what advantage is it? Heres why I ask. I already have
a Premiere PayPal account, and with it, I already set up PayPal
billing on my site, via the "Subscribe"