If you are going to photograph a subject with a lot of magnification and that is moving, it is easy to lose sight of it. For example, birds and the International Space Station. Then it is good to have a sight see more of the environment so you can quickly catch the motif again.

I use the sight to find the motif and then fine-tune with the camera sight

The sights can also be good for astrophotography.

Sights consists of two parts to be glued together. Remember to make sure it is straight and test it so that the sight matches the pictures it takes.