Long time user of Putty, like many here. I liked this quote from their FAQ (A.3.3 What's the point of the Unix port? Unix has OpenSSH):

"There were development advantages as well; porting PuTTY to Unix was a valuable path-finding effort for other future ports, and also allowed us to use the excellent Linux tool Valgrind to help with debugging, which has already improved PuTTY's stability on all platforms."