Redis is often used as a cache, although it offers a lot more than just in-memory caching (it supports persistence, for instance).
What are the reasons why one would choose to use Redis rather than the .NET MemoryCache? Persistence and data types (other than key-value pairs) come to mind, but I'm sure there must be other reasons for using an extra architectural layer (i.e. Redis).
MemoryCache is embedded in the process, hence can only be used as a plain key-value store from that process.
A separate server counterpart of MemoryCache would be memcached.
Whereas redis is a data structure server which can be hosted on other servers and can be interacted with over the network just like memcached, but redis supports a long list of complex data types and operations on them, to provide logical and intelligent caching.