Hi again, as Alan says it's because it looks nice and it doesn't chip as easily.
It can be a rust trap though - it's porous and allows water through so keep it wax polished. The quality can vary a lot too - chrome needs good copper plating underneath.
It used to be said that the chemicals required to do it eat the steel and make it thinner and brittle - many frame builders wouldn't chrome plate Reynolds 753. It was very common on Italian frames made of Columbus tubing though. It's not done much now 'cos most frames aren't steel and the chemicals cost a fortune to dispose of 'cos they're so environmentally unfriendly. Did used to look nice though.