It has the advantage of being a drop in replacement most places everyone uses v4 today. It also has the advantage over other specs of ulid in that it can be parsed easily even in languages and databases with no libraries because you just need some obvious substr replace and from_hex to extract the timestamp. Other specs typically used some custom lexically sortable base64 or something that always needed a library.
Early drafts of the spec included a few bits to increment if there were local ids generated in the same millisecond for sequencing. This was a good fit for lots of use cases like using the new ids for events generated in normal client apps. Even though it didn’t make the final spec I think it worth implementing as it doesn’t break compatibility