From https://en.wikipedia.org/wiki/BBC_Micro "the height of the graphics display was reduced to 200 scan lines to suit NTSC TVs". But NTSC is supposed to have 241 visible scan lines per half frame. Why wouldn't you want to make the graphics display vertical resolution 240 instead of 200?
While nominally 241 scan lines were visible in the sense they contained video information, all TV sets hid a varying amount of scan lines on top and bottom (and left and right) by overscan and by the bezel in front of the screen.
So with a vertical resolution of 240, on most TV sets parts at the top and bottom would not be seen. While this doesn't matter much for movies, it's not a good thing if you want to do text editing.
This is also the reason while basically all homecomputers and game consoles had some sort of border (which often could be colored) around the center part of the image that carried information: It was to make sure this central part would be visible on all TV sets.