It's not necessarily a question of each track having the exact same characteristics - that's silly Har Bal talk
. But there is often a desire to have a level of coherency, an impression that the songs belong together: an "album" is, by definition, different from a simple compilation, and usually there is a desire for some kind of natural flow from one song to the next. This doesn't mean that they have to sound the same, but they (usually) should not sound like they come from entirely different albums, either. Sometimes just a little nudge here or there is all one needs to accomplish that, and that little nudge is actually easier and *better* to perform on the 2mix than to try and go back and synth it by individually re-mixing every individual track.
The problem with this thread right now is that there are people talking apples, oranges and peaches. There are those talking about mastering a single, others talking about mastering an album, and yet others talking about mastering as a means of mixing. The answer is different for all three.
Mastering as a means of mixing is baloney and should be avoided whenever possible; there one needs to get the mix right and actually do the mixing in the mixing stage.
Mastering a single (e.g. for meSpace release or as a one hit wonder) does not require any kind of "glue" or continuity mastering, and as such should not require EQ to make it sound good. But I still make the exception that I'm surprised nobody has commented on one way or the other of EQ required to keep the ubiquitous mastering compression/limiting in line (then again, that's what many people here seem to want to use MBCs for instead.)
Mastering an album, however, IMHO not only allows for, but often requires at least some EQing both for the compression control and for the "glue" across tracks.
G.