For users: because it'll spread your contribution between people who made the content you consume, without individually selecting them and managing that distribution over time. It'll automatically distribute it between sites you visit.
It doesn't handle the exact same thing as Patreon, nor are they mutual exclusive.
A sale/donations says "this is cool." A subscription says "I want to see more of this." For the former, there are sites that popped up and got better established while flattr was twiddling its thumbs for 7 years. If I were going to ask for donations, I'd post a ko-fi link or a bitcoin address, not ask people to install an extension.
Speaking as someone who used flattr for years before I heard of patreon, this question sounds rather backwards to me.
That said: I’m pretty sure I dislike the changes done here, and it will definitely cause me to reconsider my usage of flattr as a whole.