Robin's keeping it casual! Sometimes tights, body armor and all that other crime fighting gear gets in the way of feeling comfortable while taking on super villains. This costume solves that problem by creating a t-shirt version his uniform.
Getting a superhero physique is tough work. You have to spend every waking moment training for crime fighting. That might be easy for The Boy Wonder, who has a billionaire legal guardian, but chances are, you're a little too busy to get buff. This look takes care of the whole "muscle" thing for you!
Sometimes you just can't beat a classic! Robin's had tons of different reiterations of his look, but this is the one that started it all.
This version of the Boy Wonder's look drops some of the "boy" part. It comes from the Batman and Robin movie, but even in the comics, Dick Grayson has adopted this look for his Nightwing costume.