"You open the shell to see what's inside. If you're lucky, you find a pe[a]rl. " 😎
(But beware of the daemons, zombies, ELFs, DWARFs, pythons, GNUs and other funny creatures...)
From a user perspective, the shell (text or graphical) is the most fundamental process that needs to run after boot.
It allows to observe, start and kill other processes. (And by extension: files)