Very good question! You are indeed free to copy the rendered filters to your uBlock settings (there is even a button for this). I see two reasons for using the account feature:

- I would never sync the same Firefox profile on my personal and work laptops, with letsblock.it I can use the same custom list on different Firefox profiles

- Rules will eventually break when websites are updated / redesigned. When that happens, one contributor can PR the fix for everyone, and you'll get it in the next 24h (uBlock Origin's minimal filter list refresh rate)