The best explanation I've seen for that is that they are just collecting the content in case it is needed later. If, for example, they see you making a lot of long-distance phone calls to numbers known to be ties to Al Qaeda, then they could 'get a warrant' to actually look at the PRISM data they've collected.
In real life terms, it's like posting an NSA agent in every house in America, who promises to keep his eyes closed. Of course, he can hear everything you're saying, but he promises not to tell anyone unless you say something elsewhere that would give them cause to.
The Verizon FISC order addresses only metadata. The PRISM presentation indicates that among the data captured from the providers involved in PRISM is VOIP data, which, would seem to be call contents.