Redis is a remote data structure server. It is certainly slower than just storing the data in local memory (since it involves socket roundtrips to fetch/store the data). However, it also brings some interesting properties:
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Redis can be accessed by all the processes of your applications, possibly running on several nodes (something local memory cannot achieve).
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Redis memory storage is quite efficient, and done in a separate process. If the application runs on a platform whose memory is garbage collected (node.js, java, etc …), it allows handling a much bigger memory cache/store. In practice, very large heaps do not perform well with garbage collected languages.
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Redis can persist the data on disk if needed.
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Redis is a bit more than a simple cache: it provides various data structures, various item eviction policies, blocking queues, pub/sub, atomicity, Lua scripting, etc …
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Redis can replicate its activity with a master/slave mechanism in order to implement high-availability.
Basically, if you need your application to scale on several nodes sharing the same data, then something like Redis (or any other remote key/value store) will be required.