for a very good mechanical reason in the very early days of horology and it was NOT because it looks pretty.
In the days when watches were not in sealed cases (because they had not yet been invented) and humidity and dampness were a potential problem eventually causing the steel to corrode or rust.
When steel is black polished the surface is dead flat with no pits or scratches for humidity to adhere to or be captured in and cause corission, black polishing makes it more difficult for moisture to wreak it's havoc upon steel.
Consider black polishing an early form of anti-corission resistance.
Naturally black polishing quickly became a sign of superior skill and craftsmanship and has been extensively used from the early 18th Century up to today.