Square bridges were built into some Mausers in order to simplify mounting scopes ( for double square bridges ) and to install receiver sights on single bridge actions. Since these were primarily European rifles, and claw mounts were in favor, the square bridges no doubt were a help.
The down side to them is that they added a little weight ( actually not much of a detraction on the heavy calibers) and made iron sighted rifles more blocky and a little less comfortable to carry.
Now days they are a bit of an anachronism ( there are some neat scope mounts currently made to fit them - like Joe Smithsons') but they still hold a certain classic appeal on large bore African styled rifles.