Normally? The only program I use regularly which needs function keys is htop. The only other I can think of is gitk, and I try to avoid that thing.
MacOS/OSX doesn't really have a history of using function keys, so pretty much the only software which does is stuff which was "ported" from other unices where relying on function keys is more standard.
Xcode (a macOS exclusive) uses function keys by default (for debugging). Cross-platform apps (not just ports) very commonly use F keys (eg. JetBrains stuff).
Easily. Even as a full time software developer (full stack, SQL Server, C#, Typescript) I barely use the function keys. Everything's bound to chords I can easily hit without moving my hands.
Of course, your set can be completely different, we all have our preferences and there's nothing wrong with that.
It's a good question, you made me think about how rarely do I touch that upper row of buttons on my MBP. Mostly I'm in a web browser or in terminal & vim, so I never end up using the function keys at all. What a waste.
I do it every day. The function keys to me always hid functionality, and I never found it easy to discover what they did. Usually there was another, and better documented way to do whatever they did anyway.