That’s right, ./play.it is not required in any way to play Celeste on Linux ;)
Its purpose is to improve games integration with your system, by relying on the packages manager, using system libraries instead of the shipped ones, providing menu entries, etc. In the case of Celeste, the main upsides are:
- using system-provided Mono runtime instead of the shipped ones
- using system-provided SDL2 and PNG libraries instead of the shipped ones
- work around terminfo Mono bug, cf. https://github.com/mono/mono/issues/6752
- work around Mono unpredictable behaviour with non-US locales