Why the R?

I’m watching Batman on MeTV, and I just thought of something. Robin wears a green outfit with a red vest, a yellow cape, a black mask – and a white R in a black circle on his chest.

My question: why the R? Is that to distinguish him from all the OTHER superheroes running around wearing green outfits with a red vest, a yellow cape, and a black mask?

“Hey, who WAS that masked man?”
“Was he wearing a green outfit with a red vest, a yellow cape, and a black mask?”
“He sure was!”
“You didn’t happen to notice a letter on his chest…?”
“No, sorry, I didn’t.”
“Then it could be any one of a number of superheroes. I guess we’ll never know.”