The squad size is 10 for administrative purposes. For combat operations purposes, it's actually most likely devided to 5 men squads operating very close to each other in order to have some flexibility.
Example : you are charged by Khorne Berserkers, squad would be separated in 2 5 men squads, the role of one being the first line to slow down while the second squad provides supporting fire.
First squad kills some Berserkers before dying gloriously, and 2nd squad has basically twice the time to shoot and less targets to shoot at while the first one dies gloriously.
Example 2 : you are pushing a trench held by Guardsmen, you actually want to push forward as a single unit in a narrow space to concentrate your fire and clear guys. No need and no real benefit to divide.
Military strategy is about combining and decombining the appropriate force size at the proper time and point, based on operational needs. Even at modern squads level, most militaries employ a squads made up of 2 fire teams each led by a fire team leader. During maneuver and logistics, the sergeant doesn't need to have the 2 squads operating independently, but during combat scenarios it may be beneficial.
I'm more familiar with antique strategies, so if I said some stupid stuff above, I'll let real modern soldiers correct me