Most of the time, this type of index is only useful for index scans against those columns.
The index tree structure above the leaf level will usually be very similar in terms of tree levels, so if that's the case, there's little or no performance difference in navigating the tree for seek operations (that don't also scan). If you want to learn the basics of index internals, I have a video here that will help you.
Anyways, back to the scans. The reasons why this index would be useful are much less about the index itself, and more about why not to scan the clustered index (or keep those data pages in memory). If the other columns make the table relatively wide (based on the sizes you gave, I would say they do, although it's unclear how large this table is in the context of the entire database), the narrow index may be of benefit in some cases. Frequent scans (or seeks on many different key values) will tend to keep the entire index in memory, so the larger the index, the less memory space there will be for other objects, and of course this will incur more I/O if the buffer pool is cold.
Although it's somewhat of a rare use case (I've done it once or maybe twice that I can remember), the biggest benefit of this type of index is it will greatly help queries that want to use a merge join to join this table. (Note: optimize the query first if an index scan isn't appropriate!) In the query plan, a scan of the clustered index, or a scan and sort of a nonclustered index before a merge join can give an indication of when this might be a good idea. But again, this is rare. The query should be executed frequently enough, or the base table expensive enough to keep stuck in memory (i.e., you don't need it there all the time), to justify the extra storage and maintenance for the index.
If you're unsure how this index is being used right now, check the DMV
sys.dm_db_index_usage_stats. Bear in mind that the counts are reset on server restart, so if you're considering dropping the index, make sure it isn't useful during the entire business cycle.