Is there a technical reason why the Arduino Uno has such a distinctive board shape?
It's hard to find a explanation, but, as engineer, I believe in two possibilities:

Mechanical, because for better fixation. You may then ask me, 'why, then, are there not only three holes, instead of four?' Again for mechanical reasons. Only three holes would leave the edges of the board susceptible to bend and risk physical breakage (breaking). With four holes, better stability in fixing and prevention of mechanical failures.

If the hole in the lower right corner was placed farther to the left, it would cut the track of analog channel 5. Therefore, for reasons of space, I believe the hole has stayed there.