Two monitors as a standard being the assumption. Lots of people don't have 'em, especially back when workspaces were first gaining traction. Also note that it's not particularly practical to do intensive window shuffling on tiling WMs.

When you take a break from, e.g. gaming, you would probably want all the related windows to be out of sight. Rather than close all of them and open a new batch for work then later close all the windows and reopen all of the gaming ones, you can simply switch between workspaces.

Berry seems to be a stacking WM, but there doesn't seem to be any way to minimize or otherwise access opened windows from a centralized "task bar" either, so similar workflows apply.