LAdder logic diagram-variables

FOr what is the used of the variables in ladder logic diagrams?

They are 'user friendly' names for memory locations.

For what is the use of the variables in ladder logic diagrams?

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Without any variables, everything would have to be "fixed" - not changeable and thus not programmable. That would defeat the entire purpose of using a programmable logic controller.

Note that ladder logic diagrams do not have to apply only to programmable logic controllers. Ladder diagrams can also be used for relay panels, in which case, they usually do not contain variables.