If you want to know how much you are spending on a certain type of item, you can quickly see all transactions assigned to the associated category or transfer account in the Transaction Usage snapshot. You can simply delete unused categories that you don't intend to start using in the future. Note that you can merge only categories that have transactions associated with them.
If you start off using two similar categories and then later decide that you need only one of them, you can merge the categories so that you don't lose any information that you've already entered. If you want to change categories assigned to specific transactions, recategorize them.