It's better to understand the difference between reinstalling sccm client and repairing sccm client. When you reinstall a client, it creates SMS classes in WBEM repository, initiates policies for notification to SCCM management point and gets new SMS GUID as an identification but it won't delete the exisitng GUID (SMS keeps it until we delete certificate using ccmdelcert.exe from the toolkit.. or uninstall sccm client porperly). SMS identifies it as new record and acts on it as new client. If you have added this machine to somewhere your system based collection, it won't be getting any policies. Machine, having old GUID certificate, would be negligable and policies would be rejected from respective Management point.
While, if you repair a client; it removes old certs, assigns it new certificate and initiates policies as unique record. It works properly and policies are apporved by MP.