Drop pipeline is an explicit cleanup. It is a request for the pipeline to remove a variable from the available list of variables and make the object it refers to available for garbage collection by the Java Virtual Machine.
Delete is purely a design time operation to remove the variable from the current view. It is only of use if you have created a variable that you didn't mean to create. If you delete a variable that was there because it was previously in the pipeline when you change the view in developer you will see the variable appear again.