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.

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.