I'm not a smoker but i'm sure that for some smokers being unable to light up for a lengthy flight and wait in the terminal can be very stressful. No one is going to start smoking because of a small smoking booth at the airport.

Remember that smoking is banned on airplanes, yet the minimum equipment list requires airplanes to have at least one ashtray by each set of bathrooms. Because the airplane safety regulators assume that someone will break the rules and light up, so at least this way there is a safe place to extinguish it. There's always that one in 100,000 flyer who just doesnt understand the severity of lighting up on a plane. So better that they have a legitimate place to smoke before and after their flights instead of getting so stressed that they light up in secret on the flight.