It's just the simplest way to start the conversation, a first approximation you then refine. If you explain yourself too generally, nothing sticks in the audience's head.

YouTube was "flickr for video", which turned out to be gross understatement of the community potential. Even in good cases, they are horrible analogies.

note: you're not trying to be perfectly accurate, and or flowery.

you're trying to communicate plainly, and to keep their attention long enough to get to the next slide / next pitch.

thus, sometimes taking a shortcut is imperfect, but useful.

note: "X for Y" does NOT work very well if neither X nor Y are well understood for the audience. however, it can work ok as shorthand while the person "peels the onion" and gets a deeper, more comprehensive understanding of the concept.

The best thing about "X for Y" is that it is 3 words long. If there is a more descriptive 3 or 4 word phrase, use it. Tipjoy could have been "Digg with Cash" but both "Makes micropayments work" and "Simple, social payments" are more descriptive.

They can be poorly written, but in Crossing the Chasm http://www.amazon.com/Crossing-Chasm-Marketing-High-Tech-Mai..., Geoffrey Moore suggests you use a phrase like this when trying to break into main stream. Doing so provides the listener with a "product alternative" beacon that helps them mentally position your offering.