If it weren't for reprints in single issue form I may never have know Groot from Where Monsters Dwell, not knowing it was originally in Tales to Astonish. I also have some Marvel Superheroes (15, that Farrar posted being the first) as well as some Marvel Tales my grandmother gave me. I also might not have know that Fantasy Masterpieces were reprints if the LCS owner hadn't told me specifically about them, to save me money on reading the first Silver Surfer series.

I don't remember, but did older issues like that make any references to being reprints or say where the material originally came from? If I remember right, a lot of reprints in the 90's when I was collecting, like Classic X-Men would make reference to the original material in the little copyright and print date paragraph.