You're not wrong about anything.

   Is this a technology I would only use when building websites at scale or something? 

Basically, though I'd amend that to read "websites that might need to scale at some point."

For a lot of projects, shared memory is a perfect choice. But if you might need to scale up to multiple instances at some point, shared memory obviously won't do. And since it's not like there's a huge penalty for using Redis, a lot of times it's just convenient to use it from the beginning.