Easier to access / run things on remote systems.

Possible to create tools for use on small systems -- your DSL/Cable modem, say, which is really Just Another Linux Box, though with tiny capacity.

Smaller VMs -- less system load, more available, greater flexibility.

Run a shell on your Android or iOS device. Full power of the shell, scripting, and development tools.

You can do more of whatever it is you want, faster, using less. It's an efficiency thing.

Natted VPS's are usually very cheap (~$5/year) are low on resources by design and are great for pure terminal workflows.