When I was a kid in the 1980s, the conventional wisdom was that the signature ruined the value of the card being signed. Better to sign a ball or a picture or an index card and then put the card and the signature side by side.
That all changed for me when I got my first signed T206 of Rube Marquard in 2004 on ebay. It was -- and remains -- really compelling to me. 44 signed T206 cards later...
And don't get me even started on signed T201s...