1) In some studios like the one I am working in the the animators work in shifts and sometimes on same projects. Storyboards help the animators/compositors to coordinate their work.
2) It also helps them to keep the original story intact in their mind (we all forget things sometimes). Believe me you don’t want to learn the things the hard way, luckily I didn’t either. But I did forget one small detail which I realized as soon as I saw my drawings. Never think that your better than any storyboard.
3) Do not limit your drawings because the animation is just 4-9 seconds of promo or motion graphic keep it so detailed that those pieces of drawings are enough to someelse to copy.
4) Since you made them so detailed and anyone can make a movie out of it keep it for your eyes only and your animation/graphic department.
5) Always go for better framing even if its difficult to draw. I have reasons to believe people skip drawing a good frame and later do that camera angle in 3D because its easy.
6) Make backgrounds I skipped it for now. I realized now if i made backgrounds in first place I’d have more fans in workplace and compositor/designer will have time to look for/make graphics.

Here are some printable PDF templates of storyboards
4:3
16:9

Its something I had not seen before like a dream, surreal.
Ryan on IMDB.com
Ryan Larkin on Wikipedia