Mcrouter is a memcached / redis protocol router. Like twemproxy, Mcrouter enables connection pooling and automatic sharding of data across multiple servers. Unlike twemproxy, it provides facilities for replicating hot data to multiple servers / pools, and for key-prefix-based routing.
To see how this is valuable, consider the parser cache: parser cache entries represent immutable content objects, so they don't need to be updated or purged. With prefix-based routing and replication, we could have SET operations on parser cache objects replicated to codfw, so that its parser cache is kept perennially warm. (We would need to change some key conventions, since parser cache entries are prefixed with the wiki ID, followed by 'pcache'; we'd need it the other way around.)