Telescope focal point and inversion
The main mirror of a telescope forms an image at the prime focus. 

You see the lines being brought to a single point, but that doesn't mean all the light entering the telescope is focused to a single point. It means that all the light from a single point in space is focussed at a single point. Light from different sources is focussed at different points, these points form a plane in the body of the telescope, the "focal plane".

This image can be seen. You can project an image using a single lens, or if you hold up a lens at arm's length and look through it, you will see a small inverted image. The purpose of the eyepiece is to act as a magnifying glass and make this small inverted image appear bigger.

The image is in focus only at the focal plane. If a sensor (without any further lens) is placed at any other point, no image will be seen, or at best a blurry image. We can see the image when we look through the eyepiece only because we have another lens (the cornea of our eye) bringing the light to a second focus on our retina.