What is the usage of "Reset previous"/ "Restore default"?

Reset previous:
The "Reset previous" button is shown whenever you edit panel. It helps to revert all the changes you did in the current editing session to the original state (which were saved in the last time).

Restore default:
If you click on the "Restore default" button, the column list will restore to the set of default columns (defined by the app): Key, Summary, Status, Assignee.