You need Real Player to watch Real encoded movies. You need Quicktime to watch Sorenson encoded quicktime movies. These two companies will not allow their codecs to interoperate with other players, forcing you to download their bloatware if you want to watch movies in those formats.
As for other movies in open formats (avi, non-Sorenson mov, mpeg, DivX, etc) there are a whole bunch of players that you can use, Media Player just being one of many. But since Media Player is already there, and it works fine, why bother with anything else.
There are also a whole bunch of other rare formats, for which you need codecs or specialized players. For the long-gone-but-not-quite-forgotten Vivo format, I recommend VivTV which is currently in version 4. You can download a codec to enable Windows Media Player to play .m2v (Mpeg 2) movies. And so it goes.