There are several lines of thought on this. "Weedicides", or herbicides are traditionally used commercially by farmers to maximize their crop production by eliminating or reducing weed populations amongst their crops. These unwanted plants were previously removed by large tractors with implements called "cultivators". The problem there was they didn't remove all the weeds, plus the weeds came back. Herbicidal use didn't disturb the ground and produced a higher mortality rate among the weeds.
The basic reason for this is economic. As the previous post says, weeds use up resources, taking them from the desirable plants. This lowers the yields of the desirable plants. It is too expensive and difficult to hire people to manually remove the weeds. Therefore chemical herbicides are used.
If you mean herbicides, they are used because weeds absorb nutrients taking them from the crops growing right beside them. They also absorb water and if they grow tall and fast, they can even compete for light. Therefore, herbicides are used to control weeds as they are considered "pests" even though they too, are plants.