Worn and over-used dies can result in all types of anomalies but we’re referring to dies in which mint employees attempted to prolong the life of the die and inadvertently filed off design features. In return this created some valuable die varieties that are highly collected today. Not to be confused with Omitted Mint Mark.
However, not all missing design features make a coin valuable, there’s many instances of a coin being struck through grease or over-filed to remove clash marks but no extra value is added.
It’s all about demand and if there’s two or more just alike and if it is sent in to be attributed by a professional with a press follow-up.