UDS can be permissioned using standard unix USER/GROUP which adds more flexible security structure (more so when you combine UDS with SE Linux).
This is opposed to localhost which is much harder to firewall from-localhost->to-localhost
The reason is that there is just too much surface between an application and the operating system to be able to guarantee safety.
And so we put applications into their separate VMs or containers and treat the entire container as part of the application. There are no other assets on that container.
Of course, if the container boundary isn't meaningful, you've complicated your system design for nothing.
And either way, TLS termination in a separate process is a potentially significant penalty to performance, as you'll have a lot more copies.
I can see how it sometimes may make the application simpler and maybe, potentially, how you can have single proxy to serve multiple tenants (containers with applications) so that they maybe don't have to keep their own pools of connections.
The services would talk to each other locally and use the proxy to talk to the database using single connection pool.
But I don't see how this couldn't be as effective with TCP loopback (which actually is hacked to not invoke any TCP).
Granted, domain sockets are bit faster than localhost (https://redis.io/topics/benchmarks) but if this makes difference for your application you are likely using wrong architecture.