It adds an extra 5 horsepower and knocks 3 miles off per gallon.
Seriously, though, I did put a TRD decal on my doors for a while. It was a bit of a joke. My then-girlfriend, now-wife made fun of me when I took her with me to go look at a 2002 4Runner to decide whether or not to buy it. The dealer was located in Rockville, MD about an hour away from where I lived in DC. She was busting my balls and telling me the only reason to buy the 4Runner was to impress the ladies of Rockville to see if I could hook up with them and wind up becoming a Rockville daddy. The entire test drive she insisted the 4Runner would have to live out its life as a Rockville daddy's car.
That was 2012 and the name stuck for the old boy. He's been known as The Rockville Daddy ever since. The initials fortuitously remain TRD, but I shortened the name to Artie from RD.
So there you have it. I guarantee you no one else you run into will have the same reason for putting a TRD sticker on the side of their rig.