You are so right for the thread-safety, immutability is a major improvements. However, I am pretty sure OP was referring to immutability in javascript, which is

You are so right for the thread-safety, immutability is a major improvements. However, I am pretty sure OP was referring to immutability in javascript, which is single-thread by design, and yet many javascript developers use immutability like a no-brainer and is seen as best-practice to the point where people don't even remember what it solves. Just that it must be used.