To answer the OP's question;
Speed; Quick Connect Faucet sets can be I installed a lot faster, reducing labor costs for contractors.
Convenience; You don't need to be a skilled journeyman plumber to get it right; "They are so simple, almost anyone who can follow the directions in a comic book can do it.
Reliability; IMHO, they are almost, but not quite, impossible to get wrong. works fine every time.