You misunderstand. The NSA has collected massive amounts of data on everyone, they could easily solve many murders, burglaries and most other crimes. But they don't, in fact they don't event really stop terrorists.

So why collect all this data? They do it so they have a massive backlog of information, so that when someone decides you need to be taken care of they no longer have to figure out something to pin on you, they don't have to navigate around that pesky 4th Amendment. The purpose is so that when they decide you need to be taken care of they can go through the backlog find whatever they want and then boom. You're done.