A tyre’s grip is determined by many factors such as rubber compounds used, the tread pattern, foot print size, an the ability to evacuate water from under the contact patch. As the tyre wears down, the ability to evacuate water also goes down. By restoring the main rain grooves up to 4mm, you also restore the ability to channel the water out and away from the footprint.
Regrooving can also restore some of the lateral grooves that get worn away towards the end of tyre life. This provides new biting edges thus regaining grip performance lost when a tyre wears down.