I guess if you want to equalize water levels in the two tanks you just open the valve in the connecting pipe. Also if you want to fill at a rate higher than the vents of one tank is designed for you could fill both tanks at same time with 1/2 water level rise per unit time and vent rate shared between the two tanks. If I recall the first tanks in a waste water treatment plant that the influent enters are called equalizing tanks connected in such manner. Their is a process reason for this that I forgot. but since they are just storage buffer tanks I think the purpose is so both tanks contain approx. same mixture since influent content can vary widely. Search "equalization tanks" on internet..
OP, There are so many reasons why tanks are interconnected. Ask AI and it will tell you.
Another design case is limiting the fire or explosion, in case a tank goes on fire. It will allow to reduce the inventory by transferring it to other tanks.