It seems to me that this is a bridge between Redshift/S3, so you can join data from both sources. I believe Athena is S3 only

If you are already invested in Redshift this is a really nice feature to have.

I used to work in an environment where only data for the last few months was stored in Redshift (to save costs since storage is expensive). Whenever someone need old data, we need to make room for it unloading tables to S3. Now this is not needed anymore, and is is awesome.

Another nice reason is that some BI tools doenst work with Athena (and PrestoDB) yet (Metabase and Pentaho for instance), so it is now viable to use Redshift to expose data inside S3 to these tools.