Metal Store

Metal stores gives you extra Metal cap, meaning you'll have less floating Resources.

Main use
It simply gives you more maximum storage, which means you'll be able to have more Metal stored at a given time, letting you accumulate some.

Sometimes you need metal stores just to be able to put down some buildings, as they may be too expensive, requiring some storage.

Other info
They are essential in a Plasma Laser Rush, since you need to build 1 extra metal store and 1 extra Battery (compared to what you start with) in order to be able to store 1000 metal and 6000 energy to build the laser.

An exception to this is Architect, as she allows you to have 1050 metal storage and 6000 energy storage at the start, on a standard map where you have 1 metal store and 1 Battery already built.