A mower can only do so much. It can't get the grass growing flush against the fence line. It can't get the weeds in your flower bed.
String trimmers trim down what your lawn mower can't, and gas trimmers boast great power and versatility. They can be used for edging, separating your lawn from your walkways.
Gas-powered string trimmers also give you the freedom you need to move around a large property, around trees and other obstacles, without becoming tangled by an electrical cord.