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The biggest thing to me about BTCPay is the sovereignty of your funds. Instead of using a third party you can retain ownership of your funds 100% of the time since its just tracking your wallet addresses. This also relieves any fees that a potential processor may attempt to take for acting on your behalf but sovereignty is the real draw to me. (Plus the ease of use on setting up a lightning node for those kinds of payments but that's a whole different topic from the underlying need.)

I believe you are mixing payment processor with payment service provider (Eg. Coinpayments vs Bitpay). This eliminates your need to use a third party for processing and thus remove the counterparty risk as well as any restrictions they may hold against your business type (Wikileaks). If you wanted a payment processor you could still sign up for one and set your end address to automatically forward to the payment service provider.

For example my site I have it set to accept payments through Coinpayments.net so that once the payment is made it will automatically update the order once my preferences are met and allow the customer to receive their order immediately. This allows the store to act autonomously but I'm still relying on Coinpayments plus I have their fees. If someone wrote Steem into BTCPay I wouldn't need to rely on Coinpayments anymore and could just accept the Steem into my own wallet and have a second bot that monitored my address and sent funds to say Blocktrades to immediately convert to BTC which then would be sent to Wyse, Bitpay or Coinbase for conversion to USD. Now my payment service provider only knows that I'm wanting to sell my BTC but not where it came from or why I obtained it (outside of their public chain analysis.)

EDIT: I guess to get to the final question the whole point is to allow your backend system to be able to monitor the chain without reliance. BTCPay functions very similar to BitPay's services (without the fiat option) so it can be adopted by most services fairly easily to allow them the option to not rely on Bitpay. Additionally it means I don't have to deal with Bitpay trying to push BIP70 down my throat while telling my customers that they will have to pay huge fees with BTC and not BCash.

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