Noop is a command that does literally nothing and doesn't care about any parameters.
What is it good for, then?
It is a handy command when you need to evaluate some identifier without using the actual result. For example, you may want to evaluate a $regex identifier without using the information how many results it returns, just to later use the back-references it creates. Another example is $findfile which has a command option in itself, and therefore doesn't need any outside commands to handle its results.