It's the building structure too, they're easier to design and construct using a repeating structural module. Each column supports the same area, each area has the same design loads, and you get a bunch of identical columns in a line.

And then you need to arrange rooms and corridors around those columns, and it's much more convenient to not have random columns dropping through the middle of most rooms. So you align your walls on the column grid, and now you have a bunch of straight walls and 90 degree angles.

To some degree it goes the other way (I need my rooms to be X by Y, design column grid to match), and you still do rectangular rooms because you know that way you can push your column gridlines into the walls. And it's not perfectly consistent, your central corridor might fall in an off-module grid width. But one way or another most modern buildings are on a grid like that.