I can't add much to anything else said here except that I found a nice sash fillester at one point and was sure that I'd never find a use for it, but I couldn't pass up the deal.
Two years ago, I had to make a lending library for the neighborhood and ...well, I ended up using the sash fillester for the door. It wasn't fine work, I wouldn't have needed to have done that, but I can see why someone making sash would love having a constant thickness of stock left on the inside.
(looks like my door is warping.. that may provide a second opportunity to use the plane).
The neighborhood ladies club painted and put a mosaic in this one (we got a free smaller one along with this one, I had to copy the style of the free one, so criticism of the design won't hurt my feelings!!). They did a nice job than I did, but this may be the only lending library made with hand tools. Even the dadoes in the ply were cut with a trimmed down dado plane).