Why do tires have grooves?
Not all tyres have grooves. Take the "slick" tyre - it does not
have grooves. It is used always in racing (as it is illegal to use
on most countries roads) and only in dry weather (permitting).
"Normal" tyres have grooves, as a channel to expel water from
the surface of the tyre. Without these "grooves", driving in the
wet would be a hair-raising experience, as the tyre would aquaplane
on the water giving the driver, no grip or control.
This is also the reason that most countries have a limit on the
tread depth, to ensure that the channels are large enough to expel
any water away from the surface of the tyre.