Magnifiers can be attached to your viewfinder to magnify or enlarge the image you are looking at. They can also be used for digital cameras to magnify the LCD image. This makes manual focusing a lot easier. They can enlarge the central part of an image on screen by doubling the magnification or more of the viewfinder you are using. Some magnifiers can be swung on and off by a hinge between focusing and viewing the entire image.
Most magnifiers also have adjustable diopters built in them. You can also get cases for magnifiers to protect them and carry them in. Because you don't have to press your eye against the viewfinder, magnifiers can help to alleviate strain on your eyes and can also help prevent injury if you are covering sports events or demonstrations, etc. You can raise or lower the camera for focused shots and still keep the full frame in view. Magnifiers range in price from about $40 to several hundred.