Let's say you have 2 tracks, one analyzed at -14db and one analyzed at -10db. When played back with replay gain turned on, MJ will adjust the volume by those amounts resulting in them sounding about the same volume. The problem is that they'll both be a lot softer than they were originally. To compensate for this, you can set the the global adjustment up to bring them both up to a more "normal" level (the definition of which is left to the listener). In this example, if you set the global adjustment to +10, the second track would sound about like what it did before and the first track would be set to -4db to make it close.
There's at least one serious downside to this however. Tracks that aren't analyzed default to 0db adjustment which means if the global adjustment were set to +10db you'd probably have to pull the cat off the ceiling when one of these tracks played.
Most of my music analyzes at between -14db and -7db so I have my global set to +6.