Mic level is the direct output level from the mic. This is the amount of electricity created by the microphone diaphragm, and is a fairly weak signal.
The gain on the mixer is a pre-amplifier which makes the signal stronger so it can be dealt with by the mixer. Once it has been pre-amplified by the gain, it is at
line level. This is the level that is sent to all the
compressors, and any processors, and that is eventually sent to your speaker amplifiers.
Other inputs like CD's are already at line level, and these usually have RCA or 1/4" jacks. So, it is kinda the standard that
XLR is pretty much only used for microphones, which always need to be boosted from mic level to line level.