In which cases does it make sense to use an explanatory video on a homepage?

I am a junior designer and I working on a web service's site design. I am not sure what should I place at the homepage i.e.:

Some services like dropbox and readability have a quick video tour which explains what the product is and what it does. While others, like BasecampHQ and Campaign Monitor, have just static images and text.

How should I decide what to use? Are there any general guidelines? or does it all depend on the product?