The airbox preheat thermostat is designed to warm up incoming cold air to about 10C (50F) in cold weather situations. This is intended to reduce emissions, particularly during engine warmup when ducting, throttle body, intake manifold passages, etc, are cold. Improves fuel vaporization or such. Some have said it speeds warmup. Doubt that. I can't see that there could be any measurable difference heating up a 400lb lump of cast iron and aluminum with combustion explosions going on inside it, just by raising the incoming air temperature a little.

The airbox workaround described on this BB years ago was to insert a finishing nail of the right length in place of the thermostat. The "right" length was that which holds the flapper valve to fully block the heated air port. Then you connect the flex tubing so everything looks factory stock.

I placed the sensor of one of those wired digital thermometers in the airbox with the readout on the dash. The preheater definitely works, and very quickly after engine start. But even with a new, working thermostat device, the temperature would climb and climb when using a lot of throttle up a long hill, on a cool day. At steady cruise with light throttle it would hold 12-15C. So the thing works, but not that well. If you live in a moderate climate, or unless you're a purist...finishing nail.
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Son's XC70, daughter's XC60, my 83 244DL, 89 745 (Chev LT-1 V8), and XC60. Also '77 MGB with Chev V6, and four old motorcycles. Long gone: 1981 244, 1994 940 and 1998 S90.