There are varied reasons. In my experience the common ones are.
This used to be common in older scientific and engineering uses. The reason is that the figure is actually valuable data, the reader may need to measure stuff from the image with a ruler, apprise 2 closely aligned points that lie on a logarithmic scale. This is still somewhat commonly done by engineering analysts as its much nicer to apprise a graph at A4 size than 1/3 of an A4. The reasoning is most likely the same it makes reviewers job easier.
Second laymen, scientists, secretaties etc. dont allways use best of quality tools. Vector graphs are less common than one thinks, eps files tend to rasterize in transit etc. By requiring full size pages images have room to be scaled down. This reduces errors. This would not be such a issue if the submitters used indesign, possibly latex. But some areas use word and random plotting software from 1970'sso results arent allways top of the class.