You are correct the language really shines when there is concurrency (not necessarily just distributed systems), because validating system invariants with the model-checker is just magical. I've also found the language to hold great value for specifications as such; Dr. Lamport seems to believe that most technical problems just disappear when you precisely identify what it is you want to do, and writing a TLA+ spec is an efficient way of accomplishing that. So, I've found it useful to write specs for solutions to very technical problems simply to clarify my thinking and root out areas of sloppiness.