You mean the core client that has a complete cross-platform GUI? It's a reference implementation. Even if not many people choose to use it, it gives developers something to test against and it allows us to verify that what we've designed actually works in the real world. Plus, there are plenty of users who may choose to run a full node wallet, anyone who has a computer switched on 24/7 is a good candidate for using Bitcoin-Qt. With this question, you're really asking "why does anyone bother to maintain the Bitcoin-Qt GUI at all?" which is a topic for another time.