These ergonomics accessories are designed to alleviate the strain of leaning forwards over a flat working surface to read paper documents. If you can bring your working surface up towards you, rather than twisting and having to lean over the working surface yourself, muscle strain in the upper back, shoulders and neck can be minimised.
Older style document holders were mounted to one side or other of the monitor and there are times when these are still of use. The majority of document holders are designed to sit in-line between screen and keyboard and give both a larger surface area and the capacity to hold ring binders or other forms of document storage. Many are also angle and height adjustable.