From Diablo Wiki
A root debuff effect is very similar to a slow or snare one, with the difference that the slowing amount is 100%. The target will be rooted in one place for the duration of the skill, spell or ability.
Particularly useful against targets who try to flee, or you are trying to flee from.
A root status effect rarely means the attack speed is decreased, but it is one of the most efficient way to kite an opponent.