Screencasts seem to me like they are the right tool for explaining standard paths through complex user interfaces. This is especially true of poorly documented and/or very open ended interfaces (e.g., in my case, Emacs or Photoshop). (For example, I only realised that I could open a remote MySQL prompt through SQL-mode, using TRAMP, after watching a screencast.) That said, in the same way that people argue that good naming practices should obviate the need for many comments, I worry that people use screencasts as a crutch to avoid implementing needed usability improvements.

I think screencasts also guarantee that your thing sort of works, which adds an air of professionalism.

That said, gripes include: hard/impossible to speed up while watching, hard/impossible to bookmark a specific spot, hard/impossible to skip over bits that you already know, requires headphones/speakers/video, often hard to see what's going on, and easily goes out of date.