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@Dammit
Honestly IDK - I'm going on what I read on a forum (not this one, but this is similar info):
The freewheel was a holdover from the earlier models of Saab (92, 93, 95, 96, and Sonett II) that were powered by 3-cylinder, 2-stroke engines. The original version of the 2-stroke engine was lubricated by oil mixed with the gasoline. Think what would happen without a freewheel: You'd come to the top of a hill, take your foot off the gas, and the car would roll down, with the engine providing braking just as you'd want it to do. BUT... with the throttle closed (foot off gas) and no gasoline going through the engine, there'd also be no OIL going through the engine, so it would get no lubrication and wear out very quickly. The freewheel was designed to prevent this: When you took your foot off the gas, the engine could drop back to idle speed while the car coasted freely. (The lockout knob was provided for special situations in which you HAD to have engine braking even though engine wear was increased -- steep mountain driving, for example.)
By the time Saab switched to a more sophisticated two-stroke engine with automatic oil injection, the freewheel technically was no longer necessary... and of course, the switch to the four-stroke V4 cemented the change. However, by then Saab owners had gotten used to having a freewheel, and Saab claimed (probably correctly) that it did improve gas mileage a bit (because the engine could idle while the car coasted.) The books also said freewheel let you upshift without using the clutch, although I always got pretty disturbing crunches when I tried it!
As to the original purpose of the Sonett, I do think Saab intended it as a serious sports car, not just a "commuter" car. It was based very closely on the 96 sedan, which had quite a sporting reputation in its day as an international-caliber rally car... so by substituting a lower, lighter, two-seater body, Saab was able to provide a more stylish car with more sporting performance, based on the same well-proven chassis. In principle the idea wasn't much different from the scads of British sports cars that were based on mundane sedan mechanicals.