Use it: when joining two stranded wires of different gauges. Sure, you can buy fancy step-up or step-down connectors. (We’d love to sell you some!) But if the jump in gauge is larger than one step, you can get a much better connection with a parallel connector sized to fit correctly.
Use it: when you don’t have a lot of room or are concerned about strain at a joint. If you are stuck making a repair on a harness connector that needs to be replaced, using parallel connectors can help you remove some of the weight and bulk on the back side of a plug. This isn’t usually the “correctest” solution, but it can be a viable option when repairing something exotic or hard to obtain, and it’s often a way better choice than trying to force a butted connection into a tighter bend than it should see.