Rust protection spray services question

Hey Tildes car folks, so Rust Protection like this one in Canada -- is it beneficial and or necessary for cars during in Canadian winter? I know we have a lot of salt on the roads, and just in the sea air as well. Talked to a tow truck driver and he always does his work and personal cars every year.

So I took mine in too. But i wish I'd known that they don't wash the car beforehand. Like an idiot I just drove to the appointment and I got my car back and it's still covered in a layer of regular road dirt. Did I just waste $180?

Bonus question: the other older car has a 5 x 10 mm rust spot on the side - would this kind of coating do anything about it or what do I do to fix it or not let it get worse?