Well technically all DMR radios need an ID number... It just does not have to be "registered".

But it must have an ID number of some form and it should be unique on that network for things to work right.

All the registry does is ensure uniqueness and enable a lookup of call sign information and similar from the ID to display on the radio for convenience.

The DMR registry is free for ham callsigns, though some places do except donations.

Since most ham DMR repeaters are networked it becomes more critical that the IDs are unique. Tens of thousands of DMR users may be active on a network at any given time.