Sometimes, marketers setup separate campaigns that (accidentally or not) contain an overlapping array of items (i.e. same items in both promotions). This can lead to undesired outcomes, such as a product getting double discounts, or not getting a discount at all (depending on exclusion rules).
There should be a built-in tool for marketers to see which products are part of multiple (overlapping) promotions. We've built one such custom tool for a customer, but such a tool should come OOTB.