Overview
A commit should represent a coherent saved change. The staging area gives you a checkpoint before permanence.
Without this intermediate step, every modified file would be committed together whether or not the changes belonged to the same task. Staging allows you to separate unrelated work and verify what will be recorded.
The command form matters because different forms have different scopes. A broad command can be efficient when all current changes belong together. A specific path is safer when only part of the working directory should be committed.