Just for reference, I found this
https://what.thedailywtf.com/uploads/files/1462690324423-upload-f8cce917-e33...
because one gets easily confused by what is called when (hopefully the diagram is correct).
Now back to what you suggest - it's not enough I'm afraid. Imagine this situation - you are on macOS and you use zsh as the default shell in your terminal. Now you need to update your PATH. Since you are using zsh in terminal, you have to put it to some file like .zshenv because .profile isn't read in this case (it is only when zsh is used as sh). Now you run Geany but there either zsh is used as sh or there's a regular sh which reads .profile which doesn't contain PATH. So now you would have to duplicate your configuration so it's both in .proile and .zshenv and people shouldn't have to do that and we should have an easy way to switch to the shell they use in their terminal.