Estimates help planning. The problem is that they often travel through a company and become immovable deadlines.
But seriously.. there's levels to planning. There's strategic planning which is less often and then implementation of delivery stages which is more on going. It is helpful to know if something will be delivered in 2 weeks or 2 months. The problem is when the dev team says 2 weeks but discovers more and knows it will be 6 weeks but the deadline is firmly set to the initial 2 week estimate.