You can have customised metadata without records management. You can define your own content types, properties and aspects or use the ones already provided.
Records management is all about the controlling the lifecycle of records. So for example in RM you can define things like Jack can create records, and Jill will review them. Then records must be kept online for 3 years" then moved to a dusty warehouse for another 6 years. Or another use case is that the company lawyer needs to find all documents relating to a certain product and make sure they are not interfered with or destroyed.