On UNIX®, the shell is the main interaction tool between the user-level and the system. That doesn't necessarily mean the user always sits infront of a shell, but it's integral part of the system, not only an "optional commandline interpreter".
The main job of a shell is to execute commands as a user requests them. This behaviour alone doesn't help much. A shell knits some intelligence and flow control around the possibility to execute commands - it's a complete commandline-oriented user-interface (UI).
See also (external)
- Wikipedia: UNIX shell