Horizontal dropouts were around before vertical drops, and they allowed much looser production tolerances. They also allow for correct chain tensioning on one-speed and internal-hub gear bikes, which used to be much more popular before the advent of cheap, reliable derailleur systems.

When replacing the wheel on a derailleur-equipped bike with horizontal dropouts, you should try to place the axle directly above the derailleur mount. Exact positioning normally isn't super-critical.