Q: Why don't manufacturers simply the mention the breadth and height of the screen since that's so much easier to visualise?
A: This method of size measurement dates from the early days of CRT television when round picture tubes were in common use, which only had one dimension that described display size. When rectangular tubes were used, the diagonal measurement of these was equivalent to the round tube's diameter, hence this was used. http://en.wikipedia.org/wiki/Visual_display_unit