The QuickTime Movie (.mov) file format, an openly-documented media container.
Developed by Apple

In other words, .mov doesn't mean it's ONE type of audio format or ONE type of video format.
It's simply a 'container' for a variety of codecs.
QuickTime format (.MOV) supports natively (to varying degrees) include AIFF, WAV, DV, MP3, and MPEG-1. With additional QuickTime Extensions, it can also support Ogg, ASF, FLV, MKV, DivX Media Format, and others.

This is the same way that Microsofts .AVI (Audio Video Interleave) works.
An .avi can contain various types of encoded audio and video in a similar way to .MOV

The point is, there are far more (useful and free) editors of the .avi container that there are the .mov container. Hence the often recommended convert to .avi first.
Just makes life easier.