The primary optical element need be polished on one side only. A mirror can be supported all across its back. A reflector has no chromatic abberation.
Some light is absorbed by the primary lens in a refractor.
Reflectors have more light gathering power than refractors of the same aperture. It is closer to the stars. Stars do not twinkle when seen from space. It can detect infrared and ultraviolet light as well as visible light. It can be used all the time, and not just at night.