SSL is a pretty important feature for almost all apps that you run in the datacenter. The idea is not to securely send Redis data to an end user on an untrusted network, the idea is to reduce the blast radius of a compromise inside your datacenter. A good example is that Slack postmortem from a couple weeks ago -- they had a proxy running inside their datacenter, and it could be convinced to make connections to internal addresses. If the service it was trying to connect to required the client to have a valid TLS certificate, the proxy would likely not provide the right credentials (because who uses client certificates on the Internet), and the connection would simply fail. A big security bug would manifest as a higher error ration in the service, instead of letting an attacker poke around in their user data. (Network based policy is also good, but is often too broad a brush. You might want the proxy to be able to talk to a database server in your network to store some results; now you can't simply add a firewall rule that says "no traffic may pass to the internal network".)
Finally, you might remember that internal NSA slideshow with the "SSL added and removed here ;-)" when talking about how they stole user data from Google's internal network. After that leak, rollout of internal mutual authentication/encryption accelerated, because people were actually inside the network observing internal RPC calls. It wasn't theoretical, it was happening.
Ultimately, mTLS is a pretty simple way to get a large increase in security. It has its disadvantages; you now have another "moving part" that can interfere with deployments and debugging (an attacker can't tcpdump, and neither can you, easily), but given how many large companies have exposed user data through unintentional interactions, it is something worth considering. It's a technique where you have to go out of your way to cause surprising behavior, and that is always good for security and reliability.
Finally, you might remember that internal NSA slideshow with the "SSL added and removed here ;-)" when talking about how they stole user data from Google's internal network. After that leak, rollout of internal mutual authentication/encryption accelerated, because people were actually inside the network observing internal RPC calls. It wasn't theoretical, it was happening.
Ultimately, mTLS is a pretty simple way to get a large increase in security. It has its disadvantages; you now have another "moving part" that can interfere with deployments and debugging (an attacker can't tcpdump, and neither can you, easily), but given how many large companies have exposed user data through unintentional interactions, it is something worth considering. It's a technique where you have to go out of your way to cause surprising behavior, and that is always good for security and reliability.