What they do....
Improve grip on rear
Improve steady braking
Reduce skids
Stop you playing on ice and car parks
Decrease your speed slightly, especially in low pressure mode
Give a "riding over gravel sound" when in high presure mode
What they don't do....
Stop wheels tracking on ruts in icy roads
Give good emergency braking (still better than MTB type tyres)
Make the general standard of driving better