Salt is hydrophile, which means it attracts water.

• To much salt (outside cell) now attracts the water in the cell to the outside -> less water in cell, cell dehydrated.
• To little salt (outside cell): salt in cell attracts water from the outside, but now salt levels in the cell are diluted (these are actually needed in your cell to function).
• Just the right amount of salt: cells can now directly use the water without diluting the salts they contain and continue working as normal

https://en.wikipedia.org/wiki/Osmosis

Evolution and shit

It's from homeostasis, homefry!