I've noticed that - I use it when I'm not in any hurry (50-60mph)
I also use it for traffic jams (it makes the car easier to control in a slow crawl.)
Contrary to popular belief, the SNOW button does actually do somthing.
The IS200 has an electronic throttle - and as such can be changed at the push of a button. So pressing SNOW, dulls the response of the throttle. AFAIK, it doesn't affect the cars ability to accelerate, it just means you have to press the accelerator harder.
The IS was made with a livley throttle, to make it feel sportier than it actually is - I would have preferred if they reversed the set up and had it dulled for normal driving, with the option to press a SPORT button to liven it up.