Disclaimer: blunder of english
Hello! so, the 9th and 10th centuries in south Italy, from my understanding were the centuries where the Lombard princes of Capua, Salerno and Benevento(how do you spell it correctly is one of life greatest mysteries), independent byzantine duchies like Gaeta, Napoli and Amalfi begun the process of building more and more fortified settlements outside the castle/city/whatever of the ruling lord. Which in turn lead to some sort of distributions of power and lose of territory for the prince/duke, how did churches/monasteries like Montecassino and Cava worked through those centuries? did they used the process of building new protected/unprotected settlements through agreements and buyouts of formerly princely and colonized new lands? were they independent from the duchies/Lombard princes?
How did it all grew even more complicated with the Normans? When Guaimar IV ruling as prince of Salerno and even Capua for a short time, he had the Normans as vassels, but it also known that Montecassino fortified their own lands due to the Norman threat. How did it worked into the 12 century as well and the formation of the kingdom of Sicily?
I think you're a bit confused with regards to a number of different concepts. First and foremost being that the foundation of a settlement and the foundation of a monastery are two different things; and besides, many monasteries and convents in Southern Italy date to the Lombard or Gothic era, which is another discussion in and of itself. Plus, self-sufficient monasteries owning large agricultural estates are really more of a Northern European phenomenon; that's not to say they didn't exist in Southern Europe, but urbanization levels (particularly in Italy) meant that urban bishoprics were the more prominent authority structures parallel to and in many cases superseding the landed aristocracy. Further, many if not most of the quirky autonomous ecclesiastic enclaves in Southern Italy you seem to be referencing were in fact instituted after the Norman conquest to curry favor with the Papacy, and most, live Benevento, were centered around an urban community.
The first part of this answer of mine from a few months ago narrates the evolution of power structures and leadership in Southern Italy. That might clear up some historical narrative.
That being said, why would the monks in Montecassino build new settlements? The monastery itself is rather isolated, atop a steep crest little over a mile from the eponymous town. The town of Cassino has evidence of settlement since the neolithic, while the abbey was probably constructed sometime in the sixth century at the site of what previously would probably have been a pagan temple. Now, Montecassino is most certainly an exceptional place, having been a prominent medieval center of learning, but let's not forget it also benefits from being eighty miles from Rome: if Montecassino hadn't been founded some other nearby community would have filled its role (this is not altogether uncommon; in Lombardy to the north, the Bishop of Milan developed a similar albeit more complicated relationship with satellite seats in Como and Pavia).
The Abbot of Montecassino fulfilled a leadership role in the nearby town, but that's more than common in medieval Italy: it was the norm. In fact, what's curious is that there aren't more urban communities in Southern Italy fusing religious and temporal authority. I also wrote about medieval Italian power dynamics here. Of course, none of the answers I linked are definitive, and feel free to ask additional questions for further discussions.