You all probably know that wild animals, especially predators, mark the borders of their territories, so that their competitors would know to keep out. "This is my territory, this is my home. Stay away." And it's not only animals that do this -- humans do it, too. It's unlikely that a picket fence will keep thieves out, but it marks the border of your territory. "It's my territory, keep out." And amazingly enough, these things work.
The situation is a bit different on the internet, in that there's not really much territory to defend. Most of the time, you're on someone else's turf. There is, however, one thing that you have to protect: your name. Your name is what counts. if someone knows it, they can hurt you. If your name and your nick are wellknown, it's easy for someone to pose as you.
Think about it. Do you have the habit of using the same handle all the time? I know that i have (even though the handle i'm using now is a deviation from the rule). I always use the same handle for logins. I use it on IRC, etc; if someone else has already taken this nick (not too probable), i use one that's close to the original one. Sometimes it happens that i register with a site, only to forget about it; however, as i use the same nick (and few different email addresses) everywhere, it's easy to recover the password. This is also how i ended up on Slashdot, by the way -- i stumbled upon it while searching for information on some thing or other, registered my nick* (simply out of habit), instantly forgetting about it, and only half a year later did i start to actively participate in the discussions.
So, i've left my mark on this thing called Gmail. Yay. Who knows, maybe some day i will actually start using it, but right now, it only serves as a sign of my existence.
* not this one, the other one.
Update: it's dead, Jim. I deleted it, as i figured that there's no point in keeping it if i'm not going to use it anyway. Yes, i would have done it even without Tomble going apeshit. Although i probably would have waited for another two or three days before doing it. Yes, i still suck. HAND.