BTCPay has more "merchant" oriented features, like generating a new address for each transaction and keeping track of them so that, for example, you can see who has paid and who hasn't. The obvious application for this is a web store, where you need to wait for payment before shipping a product; but, it could also be used in patronage scenarios where paying patrons get access to exclusive content.
Right off the bat I imagine that there is a more reliable income stream with some of the patronage sites that exist, whereas with a Bitcoin address in the footer, you are only relying on a single one-time donation.
The goal of BTCPay server is to use the Lightning Network for Bitcoin payments, which only generates single-use payment requests. Also important to point out that LN will not be functional for years, so...
You seem to be misinformed about this because LN is absolutely functional today already. What are you talking about?
Is it not functional in BTCPay server perhaps? Nevertheless, the network itself functions already and functions well. (Adoption will take time to spread, yes.)
"Absolutely functional" is when a user can send+receive a payment as effortlessly as Venmo. You cannot do this with LN at any time in the near future - routing payments, automatic channel management, sending a push payment, etc. I don't mean to bash LN but its simply disingenuous when people suggest these features are anything but pre-alpha
It's already functional and live in production for many business today. It will just keep getting better, e.x. enabling LN payments without an invoice from the person accepting the payment.