It's to do with ensuring the blocks/rails can't move, and allows easier assembly.

Relying on bolts alone means that under high loads (especially shock loads), the blocks/rails could slip. By ensuring they're mounted firmly against an edge ensures they won't move.

Also by using datum edges, it makes assembly quicker and more accurate. Most machines using linear rails will be getting machined anyway, so adding a datum edge isn't a major issue, and saves quite a bit of time in mounting and adjusting the rails/blocks. For one offs, it's probably not a major benefit, but if you're producing multiples of the same machine, that additional time mounts up.