Single click function in a Drupal behaviour?
If you don't use .once() then your code will still work but - depending on whatever else happens on the page - your click() function may be attached multiple times to the same element, which is less efficient and may cause undesired effects if, as in this case, you are using a toggle function in your callback.

Because you are limiting the scope of your selector by context, multiple attachment is less likely, but you should definitely use once() if you want your code to be robust.