Glad you asked, Desktop app is also, in a way, website. But it offers different feature set which websites doesn't offer. You have better control over you keys, you connect any websocket you want, you have realtime notifications, you have in-built image uploading, you have some of the unique features like bookmarking, image gallery, follower search, user, tag search, setting profile and cover pictures, 15+ languages for different communities etc. etc.