When do you really need a distributed cache?

Hey everyone, I've been thinking about scaling lately, especially about things like Redis or Memcached. We always hear they're super important for busy apps, but I'm wondering when they really become a 'must-have' instead of just a 'nice-to-have.' I mean, if our main database is already fast with good indexing, and we're not getting tons of extra reads, when do you decide a distributed cache is worth the extra work to manage it? Are there certain numbers you look at, or ways data is accessed that just scream 'you need a distributed cache!'? I'm trying to figure out the actual problems these things fix, instead of just using one because it's what everyone else does.