Magnifiers have just one use - to enlarge something small. A Loupe is the standard word for a high quality magnifier used in photography for viewing colour transparencies or negatives. These are still used today when a picture of the highest quality is needed.
Magnifiers vary from the folding linen tester (which is probably also one of the cheapest good quality magnifiers right up to high end magnifiers like the Schneider Kreuznach magnifier that's used my many professional Photographers (and costs a lot more !).
There are cheap but usable plastic magnifiers made by Agfa that are fine for everyday use.
Other uses for magnifiers are for Printer,Forensic Use,Viewing Insects ad Butterflies,Cloth, Diamonds,beads,precious and semi-precious stones - in fact anywhere that needs a good view of a small object. They are also useful in micro photography but for high quality a Macro Lens is usually preferred.

A lightbox is used for viewing colour transparencies or X-Rays. These come in various sizes according to needs - from an attache -sized battery lightbox up to boxes over 10 feet long.

Lightboxes are used primarily in Hospitals,pre-press and industrial and military applications for viewing transparenct or translucent objects or films. They are also widely used to inspect printed circuit boards for quality control. Hospitals mainly use Lightboxes for viewing X-Rays .