Not all of them are the size of the example shown, most of the nuraghe towers are, well, just a tower. It’s probably not that far off other ancient cultures, and of course the Romans, especially the high-end families, lived in secure facilities. It wasn’t unheard-of to have a slave whose job it was to guard the entrance, from an alcove inside the door, and that slave would be armed with the Roman equiv of a Louisville Slugger, at least, often a dagger, but would be chained to the floor or wall to keep him on duty.
One exception may have been this house, the House of the Grand Portal — the House of the Samnite is next door on one side, and apparently at some point that owner had to part with a bit of his parcel; the buyer built a narrow house in what had been the sidelot or alleyway between two grand houses, and to be impressive built a fancy entrance.
http://bellcurveoflife.blogspot.com/2009/05/herculaneum-house-of-grand-portal.html
Maybe the towers were (at least partly) grain bins (storage facilities) or silos. They appear to be well distributed, which would cut the distance to the fields, and with the distribution of one (on average) every 288 acres (a half-section of land in the western US is 320 acres) would be about right, depending on the size of the bins.