What do you use the "extra" buttons for?

Currently in the planning stage for my first diy fightstick and I'm debating how many buttons I'm going to add to it.
I'll be mostly playing retro fighters, but I plan on playing more modern titles on PC as well.

My plan was to have the standard 8 action buttons, but I see people having a couple more. Adding extra one for thumb and maybe one or two above the main 8 could be fairly simple and intuitive, but what do you use them for?

What about the extra button for left pinky I see in some leverless ones?

Thanks!