Well not all videos are in WMP (Windows Media Player) format. Some are created by a person using Apple/Mac PC's. Either that or the videos were created by using Quicktime.
It isnt necessary to have that installed on the PC but if you have any video files that are in that format you need something that will allow you to view them.
Personally i dont use Quicktime. I use the Quicktime Alternative. It is an application that i installed that allows me to view quicktime videos thru WMP.
Same thing goes for Real Player. Dont need it but if you have a video that is in that format you ned something that will play it.
Real Player Alternative
Both of those prevent having to install both Quicktime and Real Player.