You see, I find the character creation and advancement rules of the d20-system (also known as D&D 3), confusing. Sometimes you gain a feat, sometimes you can add 1 to a stat... but it's not clear to me when what happens.(*) And it's not like I've made an extensive study of the d20 ruleset, so it takes me a lot of time to chase down all the available options for the character.
Now, with PCGen, you choose a class and press a button to add a level. And then you simply get a list of things to do to level up! No more forgetting that you could get a bonus feat, no more forgetting to add skills.
We're currently playing through the Jade Regent campaign by Paizo, which uses the Pathfinder ruleset -- which I've heard described as "D&D 3.75" -- which is also supported by PCGen. Even the special traits that you can select when creating the starting character are present!
Last session, we had to start out by levelling up. Luckily, we were playing at our place, so I just booted up PCGen, pressed a few buttons and made a few selections, and I was done!
During the course of the session, I acquired a special weapon with some magical abilities. PCGen allows you to create your own equipment by taking the base item (in this case, a Longsword) and adding any modifiers you need. Then you can rename that item and simply add it to your character's inventory to equip it. And presto: all the numbers on the character sheet reflect your new awesomeness.
I also really like the fact that it's free and runs on Linux...
*: I've heard people complain about the so-called complexity of Rolemaster, but at least in RM levelling up always uses the same process. No matter if you go from level 1 to level 2, or from level 20 to level 21: you always do the exact same things.