It still increases write rate a lot over more sequential values / reduced cache hit ratio.
With sequential-ish values indexed by something like b-trees the same index pages get modified over and over again because new table rows will be in a narrow part of the index. As databases / operating systems typically buffer writes for a while, this reduced the number of writes hitting storage substantially.
Conversely, with random values, every leaf page will be modified with the same probability. That's not problem with a small index, because you'll soon dirty every page anyway. But as soon as the index gets larger you'll see many more writes hitting storage.