I see new players ask all the time why play sectorial armies over general lists. Most people respond with fire teams. Yes they give a nice advantage .Only sectorial armies may field them.
While I can't deny that fire teams are a big advantage. Even at the cost of less troops to choose from. It's not the real reason I field them.
Sectorial armies open up the availability of most its units. For me this is the number one reason for playing them. Often I find units within a Sectorial army I really enjoy and want more of them then would be allowed in a general list.
For example three of my favorite PanO units are the Bagh-mari, Akilis, and Guarda de Assalto troopers. In a general list I would be able to use a wide array of units but only 2 Bagh mari, 3 Akilis, and only 1 Guarde de Assalto.
But If I play the Acontecimento Shock army, a sectorial army of PanO I can use 4 Bagh mari, 5 Akilis, and up to 4 Guarde de Assalto. With other troops although my list of troop typesI can choose from is considerably smaller that a general PanO list.
As another example, in an Army like ALEPH I really only care for myrmodons. I just absolutely love them. With the exception if a few other units I really dont like much else ALEPH has to offer. In a general ALEPH list Im limited to just 5 myrmodons. In the steel phalanx sectorial I can have as many Myrmodons as I can budget for.