Consider that the telescope comes in to focus at a set distance behind the back of the tube. I don't know what yours is specifically, but what matters is that you can't really change this distance. The purpose of a focuser is to increase or decrease the length of the tube to allow different eyepieces and cameras to reach this focus point.
If you want to look through it visually, you'll probably be using a diagonal to hold your eyepiece. The diagonal adds a certain amount of length that the light needs to travel through to reach your eyepiece. In this case, you *probably* won't need the extension tube. However, lets say you want to mount a camera. Now that you're not using the diagonal the tube length is effectively shorter and you can't rack the focuser back enough for your image to be in focus. This is where the extension tube comes in. Now you can increase the tube length so that your camera is sharp.
I used the camera vs eyepiece comparison because this is the situation in which I most often find myself needing to add or remove an extension tube. It all depends greatly on what specific eyepieces, cameras, correctors, and flatteners you're using though. If your focus goes all the way back and it's not in focus yet, add the extension. Conversely if the opposite happens and you can't push the focus in enough, remove the tube.
Hope this helps!
