Primary job is to keep dirt and moisture away from the piston. It has another sael inside that takes care of sealing the piston.
You could assemble the caliper without the bellows but when it comes time to replace the pads you may have to dissasemble the caliper to make sure the piston is clean. If the piston is dirty, when you retract it it may damage the inner seal, causing leaks.
Hope this helps!