This is to reduce cost and in some cases is a reasonable trade of. Removable nosepieces allow you to put another set of objectives on the microscope very easily. If you are doing a lot of techniques this is a help. Cleaning objectives is also easier with a removable nosepiece. The objectives screw into the nosepiece.
Objectives provide both resolution and magnification for the microscope.
However they don't provide all the magnification or resolution. It is perfectly OK to unscrew and examine the front of objectives. In fact it is something you should do routinely. The observation tube is attached to the frame above the nosepiece. I'm not going to even go into monoculars, a modern observation tube is a binocular at least.