useless object; white elephant; boondoggle
A set phrase describing something that is not only useless but also takes up space or resources; often implies it is more trouble than it's worth. Comparable to 'white elephant' or 'boondoggle'.
This big vase is a white elephant.
The unused machine has become a useless object in the warehouse.
宝の持ち腐れ emphasizes wasted potential or unused talent, while 無用の長物 focuses on the object being a burden or nuisance.
A classical Chinese-derived phrase (無用の長物). 無用 means 'useless', 長物 means 'long object' but here refers to something that is cumbersome or in the way. The exact historical derivation is uncertain, but it has been used in Japanese as a set expression for centuries.