Mosaics are made by photographing adjacent patches of the sky and stitching them together as a single image. They are time consuming and challenging to do, especially in light polluted skies. They become necessary when an object is so large for an optical system that the object simply won't fit into the field of view of the camera. While one could simply use a camera lens to capture a larger patch of sky, resolution would be lost. It is for this reason I prefer to use a telescope and put in the hard work necessary to keep image resolution high.