Well at the speed the data comes in, you don't have time/space to store it all on disk (and just filling up also doesn't seem to beneficial for them) and also c

Well at the speed the data comes in, you don't have time/space to store it all on disk (and just filling up also doesn't seem to beneficial for them) and also can't just double all traffic for sending it to the NSA So they have to filter a lot of data for interesting bits really fast before storing / sending it anywhere else