[aimor]

When I was young and learning piano my teacher would use pencil to write in the suggested finger number above notes. Thumb=1 pinky=5. This was a good reminder when reading the sheet music of where on the keyboard my hands should be positioned.

Image search returns some examples: https://ddg.gg/?q=piano+sheet+music+fingering&ia=images&iax=...

[zarzavat]

It's also very common for string instruments, even at higher levels for difficult passages.