For Doraemon, this isn't just a decoration. In the lore of the series, the bell is a memento from his days taking care of a younger Nobita. Unable to function without it (as the loss signifies a deep emotional wound), Doraemon becomes a listless, almost catatonic machine.
When Doraemon loses his bell, Nobita realizes he has relied on gadgets for everything. At the museum, he is forced to use historical non-lethal gadgets to solve puzzles. Specifically, his skill with Shooting (marksmanship) is highlighted, as he uses a sonic blaster to disable robots without destroying them. He evolves from a gadget user to a gadget expert . doraemon movie nobitas secret gadget museum
The museum isn't just a display case; it is a living archive. It contains every single gadget ever conceptualized, from the Anywhere Door to the Bamboo-Copter , including "Phantom Gadgets"—prototypes that never made it into production due to their dangerous side effects. For Doraemon, this isn't just a decoration