When we stayed in Bologna for a month, we were told that it was used defensively. You can see a very long ways from the top of the central tower (I think it’s closed at the moment). It would have been nearly impossible for an army to approach Bologna without several days for Bologna to prepare
If you just need it to view far, then why build more than the primary tower, a backup, and perhaps a secondary backup (in a pinch, you don’t want to be caught without a secondary backup).
Same reason people build those prepper bunkers. The towers were presumably not there for the general public. Consider as well other castles and sieges. There is a limit of how many people would be allowed in, and everyone in a siege needed to pull their weight. Really, once a town is under siege, the castle is closed, when the town walls are breached, the pillaging happens and the castle does not open its gates. My point is that medieval town defense was not really a communal endeavor per se.
It's a combination of that and status. Wealthy families would build tall towers as a way to protect themselves: if you think you're at risk of getting wacked by a disgruntled competitor, you take to live in your tower for a few months, where nobody can sneak in. From that, it became a competition to show off the biggest pen1$-- sorry, tower; and then someone probably figured that the benefit of height could be used for the common good as well, installing city guards on the tallest towers.
To add to this, height advantage is enormous in combat. Arguably even more so then than today. A mob trying to storm up a narrow staircase under fire.. there is a strong force multiplier effect.
There's even a HN post about the closure! Indeed, the two most famous towers Garisenda and Asinelli are closed for renovations for the forseeable future. IIRC, they used to be the same height and had a wooden bridge connecting them at the top. From there, guards could oversee the marketplace and look out for riots.
