[dspillett]

Similarly for SQL Server sequential UUIDs can reduce fragmentation, which can save some IO, storage space, and memory load, particularly if the UUID used a table's clustering key (most cluster by their PKs by default).

Unfortunately without a bit of a hack NEWSEQUENTIALID() can only be used in TSQL as a default constraint, which can be an irritation.

And you are often better off using something else than a UUID for a table's clustering key, to help common range queries and reduce clustering key size, even if it isn't the PK or otherwise particularly unique.