That is literally how MaxPass works. Pay $10 a day to add it on.
Before the pandemic, MaxPass was $20/day/per person. But it also included Memory Maker. Also, APs could add MaxPass for an annual upcharge.
However, MaxPass did not guarantee you FastPasses in advance the way FP+ did. All MaxPass did was increase the convenience of getting a FastPass on that day. You're in Disneyland and you have a FP for Space Mountain in an hour, but two hours have passed since you pulled the FP making you eligible to pull another one, and you'd like to choose Incredicoaster in DCA as your next FP? MaxPass allows you to pull the Incredicoaster FP virtually without having to leave Disneyland, enter DCA, walk all the way back to Pixar Pier, then walk all the way back to Disneyland for Space Mountain.
However, if you didn't have MaxPass, you could still pull that second FP. You just needed to pull it in person at the kiosks.
And if DLR was crowded - which was a given - FastPasses for popular rides went quickly. Try getting a RSR FP in the afternoon most days! And once the FastPasses are gone, they're gone. So paying the $20 did not reserve you a place for the rides you most wanted to ride. That was always a toss up. It let you cut out the time spent walking from kiosk to kiosk, that's all.