Expected performance benefit of using a non-file CacheMethod (Redis, Memcache) I'm wondering what kind of performance increase, and in what situations, I might expect from using Redis or Memcache over the default file-based caching.

I know template caching always comes from the DB, so I'm not sure what else I can expect to be cached from Craft itself, and how that might impact site performance.
As with most performance related questions, the answer is "it depends". :)

Apps like Redis and Memcache store their key => value stores in memory as opposed to disk-based file caching.

Accessing data from memory is usually orders of magnitude faster than accessing data from disk.

All of these fall under "General purpose data caching" and Craft will use them internally for its own operations and plugins can certainly make use of them as well.

If you have a plugin that makes extensive use of data caching and/or you've got a heavily trafficked site, then you will likely see some tangible performance benefits.

If you're just doing a basic stock Craft install on a low trafficked site, then you likely won't see much of a difference at all.