More settlers you recruit, more food, water and power you will need. there are 30 places in Fallout 4 that contain a Workshop and consequently allow you to build stuff there or send recruited companions or people to these places (here’s how to find companions in settlements). They have a wide variety of uses and all of it begins and ends with a Workshop being present.
Settlements in Fallout 4 are places you can claim as your own to build stuff in, store items or create mini cities.