[barx1963 (Malcolm)]

Luke
You know that is a really good question!
Astro Bots answer captures the essence, but think about it this way. A telescope creates a focused image at it's own focal plane. If you held a piece of paper just above the focuser without an eyepiece in it and moved it back and forth, it would at a point produce a focused clear image. This can actually be done quite easily with the moon or a bright target. So a telescope can be thought of as analogous to a movie or slide projector
A eyepiece can be thought of as a fancy magifying glass. When you were a kid using a magnifying glass, you would have to hold it at a particular height above the ant or whatever you were examining to get it in focus.
The job of your telescope focuser is to get the the focal plane of the magnifier (eyepiece) in the same place at the focal plane of the projector (telescope) so you can examine the image created by the scope.
The focal plane of a particular EP design is determined by factors other than just it's focal length. If it was a single lense that may be the case, but no such EPs exist these days.

Malcolm