It's often used:
* When you can't recall the actual name of something: "I need to get that thingamajig from the kitchen..."
* To describe a gadget or device that seems complex or unusual: "This thingamajig is supposed to make my life easier, but I don't understand how it works."
* When you want to be playful or avoid being specific: "Can you hand me that thingamajig over there?"
Essentially, it's a catch-all word for any object that you can't or don't want to name. It's a fun and lighthearted way to communicate, and everyone understands what you mean!