help says: "Use a collection to model business logic and implement common rule engine patterns."
What if you want to process 30 decision tables in one go? You are not going to use a flow with 30 decision shapes so you can use a collection to call these 30 decision tables and process them from one utility shape in a flow.
I don't have any 30 DTs scenario on the top of my head but you could have something like:
DT1 will determine the status of your transaction (amount > 1000 = gold)
DT2 determine the status of the customer (if status = Bronze AND gold transactions in a month > 5 then status = Silver)
DT3 determines the overall discount (customer status + transaction status)