Sandbox! Not again! You just got back!
I think a skimmer removes dissolved organic chemicals (DOC), that will turn into nitrate, phosphates and silicates which will feed nuisance algaes if not exported some other way. Most people run them 24/7, and only add fish food as replacement "chemicals".
I don't think it removes electrolytes, but those are removed by your corals. You might want to teach your tank sitter how to test for the important ones, like calcium and alkalinity before you fly away. Do you add them now?
Another bonus of a skimmer is the added O2.