I originally designed the Scenario Generator for the MP games I play. Often it was very uncertain to know how much players are playing, so a given scenario might not fit. So 1) I wanted an easy way of adding more units to a scenario. 2) Players often had preferences like "only infantry", or "a small division with artillery", so the Generator allows to choose the unit types. And 3) I wanted to set the starting positions of the armies on the map. The use of 25 sectors is not perfect, but it allows to wage war on parts of the excellent existing maps you would miss out if you only use the stock sandbox deployments.