Redis improves the performance and scalability of an application that uses backend data stores heavily. It's able to process large volumes of application requests by keeping frequently accessed data in the server memory, which can be written to and read from quickly. Redis brings a critical low-latency and high-throughput data storage solution to modern applications.
When planning your Sitecore production environments please keep in mind that officially recommended Azure C1 plan is often undersized and can cause performance issues. For very small and small environments C1 is probably going to work just fine however, for medium and higher please consider selecting next level up plan. For example, official recommendation for medium is C1 -> consider C2 or higher; recommendation for 2x large is C2 -> consider C3 or higher and so on.
Here is official Sitecore recommendation: https://support.sitecore.com/kb?id=kb_article_view&sysparm_article=KB0498208
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