Every time you log in, the server checks your files to make sure that you have all of the latest patches. If you don't, it will download what's needed to make your files current.
Also, as you enter areas that you've been in BEFORE you reinstalled the game, but haven't been in since then, the game has to re-download each of these, and that can take some time.
If you're saying that the game has to completely install from scratch every time that you log in, then you have a problem and need to contact Support.
