In all the roleplaying games in all the world, you have to roleplay in mine... Someone asked me a while back: "Why GURPS?" Well, there are a couple of reasons I use GURPS pretty much exclusively, but let's ignore me for a bit. Let's assume that you've just stumbled onto my blog:
"What is this GURPS thing you speak of, O Blogger?"
"Welcome, internet wanderer. Sup from the fountain of 3d6 and know the joys of the generic and the universal."
"But I play games with inherent engines. I do not need these things."
"Oh, but you do. You simply do not know you do. Behold. The power of GURPS!"
Or some such, but more seriously. Why use GURPS at all? The other systems are arguably more popular and have a lower point of entry. You want GURPS when...
...You Want a Campaign Wholly of Your Design
You don't want to use another person's intellectual property, game setting, etc. You want to run or play in something of your own design. Optionally, you want to run or play in a setting that hasn't been adapted over to a role-playing engine. Maybe it's your favorite movie or book. Maybe it's a videogame you think is cool. Maybe you want to put yourself in the shoes of one of your favorite comic book heroes. Whatever the reason, with a little knowhow, time, and patience you can use the GURPS game engine to recreate it.
...You Want More Simulation or More Narrativistic Control
GURPS does a few things remarkably well, one of them is to simulate the real world in a way that doesn't feel...bodgy. It uses the same measurements (or conversions using those measurements) that the real world does and assigns it game stats from there. That is huge. Not many games do that. (This also aids the first point.) You can take just about anything and make it into a gameable construct.