In-memory caching is great and is super fast but remember, this is about a multilayer cache where we can play to the strengths of multiple different types of cache. Redis is fast but in-memory in the same process will always be faster.
Distributed caches like Redis have two major benefits - they allow a common cache for multiple web applications and allow a cache to survive a web application restart. An in-memory cache really just accelerates cache hits even more so.
If you had a single instance of a web app but were running both in-memory and Redis, while you would still gain performance (from in-memory), it really isn't giving you much more than what in-memory by itself was besides caches surviving web application restarts. It's when you go to multiple instances of your application - your in-memory caches are only going to have whatever is needed by that specific server whereas Redis is going to have everything for all servers.
In-memory caching is great and is super fast but remember, this is about a multilayer cache where we can play to the strengths of multiple different types of cache. Redis is fast but in-memory in the same process will always be faster.
Distributed caches like Redis have two major benefits - they allow a common cache for multiple web applications and allow a cache to survive a web application restart. An in-memory cache really just accelerates cache hits even more so.
If you had a single instance of a web app but were running both in-memory and Redis, while you would still gain performance (from in-memory), it really isn't giving you much more than what in-memory by itself was besides caches surviving web application restarts. It's when you go to multiple instances of your application - your in-memory caches are only going to have whatever is needed by that specific server whereas Redis is going to have everything for all servers.
If your curious to a big site that does in-memory caching and Redis in .NET, it is what Stack Overflow does.