This excellent question was raised by George Furnas in reaction to an early presentation of these ideas. What about a program that runs in a process controller, for example? It seems plausible that the job of a such a program isn't to represent anything, but just to do something.
But the way programs do things is by representing things. The target domain for the process control program contains the actions it controls, together with the conditions on them, and the operations in this domain include things like causing a particular action to occur under given conditions. Rather than working directly in this domain, the program permits us to carry out this operation in a computational representation domain.
This is parallel to the situation described in the previous post on composition. There is an operation on programs that represents composition of mappings, and there is an operation on programs that represents placing an action under the control of a condition. As for composition, for all this to work, the relevant programs have to have actual implementations that carry out the actual work, that is, that really carry out the actions, really make them obey the conditions, and so on.