I suspect that a reason for interception by military planes is so that, when a civilian plane accidentally-or-on-purpose flies into a "prohibited airspace", the Authorities can send a fighter to make visual contact and to visually tell the intruder to leave, instead of simply shooting them down with a Surface-to-Air Missile.
I say this because interception by a military jet is what generally seems to happen: What should you do if you are forced to enter restricted or prohibited airspace?
On the other hand I found this list of prohibited airspaces in Switzerland. Reading it, any/all prohibited areas in Switzerland seem to be prohibited only because they're a danger to intruding aircraft (e.g. because they are firing ranges), not because the Authorities are afraid of intruders (e.g. there aren't any no-fly zones over major/capital cities, as there are in some other countries).
IOW, perhaps the main threat is more to errant aircraft than to the nation. Switzerland's sleepy airforce only means that if a plane gets lost at night and wanders into a dangerous area, the Swiss cannot (except by radio) warn the errant aircraft.