In many ways, steroid use today is similar to lifting weights 50 years ago. Back then, it was more the "little" guys who started lifting. Why would a big guy need to lift weights? So naturally, that need to compenstate would carry over to the use of anything that'll give an advantage.
I'm not surprised that most users do it for cosmetic reasons. That was another thing that seemed strange to the public way back when. "Why do you need muscles?!" "And isn't lifting weights dangerous?!" Today, the general public is still confused. They don't see the appeal of steroids because they don't know any better.