I realize that this is not a new idea for mounting photographs, but I am seeing it more and more. On a recent trip to Europe, I saw it used widely at public galleries, such as the Royal Academy in London in a show that included recent works by people like Burtynsky and Chris Jordan, and at private galleries in both London and Paris.

Can someone explain what the rationale is for this system of framing? The plexiglass is invariably quite thick, about 3/8". Why is that?