i imagine he's driving somewhere using motorways, and wants to stop at some point for a rest stretch his legs and eat his sandwich somewhere other than in his car. in which case, a motorway service station is significantly more conveniant than driving down some random a road hoping for somewhere to stop.
in all honesty, i don't know. i imagine it depends on the type of seating in the service station. some have a general seating area, and some have seating areas for specific parts. at least, that's how i remember it. worst that will happen is they ask you not to eat there, and you do as suggested... buy something cheap from their shop like a bottle of coke or something.
Robh