My whole TV system is geared to "pose to pose" animation. We draw the layout poses at Spumco, then ship them to overseas studios for them to connect the poses.
When I do animation myself, my more natural tendency is to go straight ahead, or if there are poses in the layouts-to sail right past them.
I don't have the patience to draw a lot of inbetweens that look very close to the keys, so I draw all the inbetweens for fun - even if you don't see them all in the actual animation, it's more fun for me.
It alleviates the boredom (for me) of doing traditional conservative animation.
I just like doing funny drawings.
http://www.cartoonthrills.org/blog/spumco/RenStimpy/1BHB/JohnBigSleep.mov