To add value and broaden the appeal. The first version of Sparrow for Mac only supported Gmail. Even after adding IMAP, there was no reason anyone who didn't use Gmail would buy Sparrow, because everything it did differently from the standard Mac Mail app was oriented around Gmail.
Nonsense. I've been using Sparrow as my primary email client and I don't use GMail. I like the GMail-esque concept of archiving items out of your inbox and ultimately aiming for inbox zero. It works on third-party IMAP just as it does on GMail.
The primary good thing about Sparrow for the Mac is the simplicity and lack of bloat compared to Mail.app. Lack of bloat is nice regardless of what mail server you use.
