What Not to Build

If you have to show-off a new cool feature to customers without having them to download or install stuff, do a screencast.

If you need to popularize a new tool that you are using and find useful, do a screencast.

If you just prepared a new mock-up and wish to communicate it and it's purpose to your team, without collecting all of them in a room, do a screencast.

If you have a new slide-set that you'd want the readers to 'understand' and not just 'read', do a screencast.

If you wish to teach others on how to install, configure and use your product, without asking them to read through the lengthy manual, do a screencast.

In any successful project, intra-team communication is of utmost importance. And when you have globally distributed teams with time differences, it is more suitable at times to do a screencast and convey your point rather than wait for mutual free slots and discuss over telecon.
Often, putting across tutorials or new ideas in video attracts more and wider attention. And yes, voice narration is a certain cure for ambiguity.

One of the best tools in the market for producing screencasts is Camtasia Studio. A complete list of tools is here.

And here are some examples of good screencasting.