In the software world in the mid 00's, the trend started to work around the latency, cost and complexity of expensive servers and difficult databases by relying on the speed of modern networks and RAM. This started with Memcached and moved on to other solutions like Redis.
(this later evolved into NoSQL, when developers imagined that simply doing away with the complexity of databases would somehow magically remove their applications' need to do complex things... which of course it didn't, it's the same application, needing to do a complex thing, so it needs a complex solution. computers aren't magic. we have thankfully passed the hype cycle of NoSQL, and moved on to... the hype cycle for SQLite)
But the tradeoff was always working around one limitation by adding another limitation. Specifically it was avoiding the cost of big databases and the expertise to manage them, and accepting the cost of dealing with more complex cache control.
Fast forward to 2025 and databases are faster (but not a ton faster) and cheaper (but not a ton cheaper) and still have many of the same limitations (because dramatically reinventing the database would have been hard and boring, and no software developer wants to do hard and boring things, when they can do hard and fun things, or ignore the hard things with cheap hacks and pretend there is no consequence to that).
So people today just throw a cache in between the database, because 1) databases are still kind of stupid and hard (very very useful, but still stupid and hard) and 2) the problems of cache complexity can be ignored for a while, and putting off something hard/annoying/boring until later is a human's favorite thing.
No, you don't need Redis. Nobody needs Redis. It's a hack to avoid dealing with stateless applications using slow queries on an un-optimized database with no fast read replicas and connection limits. But that's normal now.