Nothing, or rather, everything.
I’m going to show you three apparently very different recipes: a traditional soup from the Centre of Italy, beans and rind; Milanese tripe, a dry soup from Lombardy; and a Neapolitan first dish, pasta with mussels and beans.
The beans are the simple link between them. The combination is less straight forward: the first two recipes mix them with offal, the third with shellfish. Another point of convergence are the tomatoes and the black pepper.
But first things first.
First, boil 250gr of beans (which you have put to soak the night before) in a pressure cooker for 30 minutes (60 or more if you use a normal pot). On the side, prepare a tomato sauce.
Alternatively, use a pressure cooker to boil the tripe or the pork rinds, or use a pan to quickly open the mussels.
When both the beans and the tomato sauce are ready, combine the sauce with the beans, season with salt and add a generous amount of pepper.
Now you can prepare a soup adding the tripe, the pork rinds, or the mussels.
If you prefer, you can make a pasta: toss short pasta cooked al dente in the beans and tomato sauce and add the mussels (in the traditional Neapolitan version), or the pork rinds, or the tripe if you feel like experimenting. Bear in mind that, for all three recipes, a sautéed mix of celery, carrot and plenty of onions will be a pleasant addition.
Also, remember that, just as the mussels can substitute the tripe or the pork rinds, other shellfish such as cuttlefish, squid and octopus can serve the same purpose. Finally, aside from beans, lentils and chickpeas can be used too, and in all these different combinations both parmesan and pecorino cheese work perfectly (even on the mussels!).
In general, every time that you find two very different things combined to the same thing, pay attention and you will discover many different ways to use that intuition.