What's the actual usecase of := in parameter replacement?

I’ve seen : "${DB_HOST:=localhost}" because it assigns the variable if not set (: $… is just to run assignments standalone). But that has the same effect like DB_HOST="${DB_HOST:-localhost} (use the string if not set), which is more readable. While something like echo "${DB_HOST:=localhost}" smells from a mile, because it has output with side-effects.