YouTube's terms of use still insist that the videos can only be accessed for streaming, "a contemporaneous digital transmission of an audiovisual work via the Internet from the YouTube Service to a user's device in such a manner that the data is intended for real-time viewing and not intended to be copied, stored, permanently downloaded, or redistributed by the user".
But things are starting to change: the videos from Barrack Obama's YouTube channel ChangeDotGov have a small download link below the player. "YouTube is rolling this out slowly, initially with content that aspires to be consistent with principles of open government. I'm told it will be offered more generally. In any case, it is an important development. There have always been hacks for slurping down YouTube videos. But it is a valuable step that YouTube encourages and supports this sharing," notes Lawrence Lessig, Professor of Law at Stanford University.
Why would you download videos? To be able to watch them without having an Internet connection, to download them faster using download accelerators like Flashget or Free Download Manager, to use some of the content in your videos or to have an offline version just in case the video is no longer available at YouTube.
