The line is the initial city design. But then in 20, 100, 500 years you have two empty sides (equally accessible from any place in the line city) which you can use for more infrastructure (e.g. airports), industry, recreation, agriculture etc.

Meanwhile if you design a disc city, you can keep expanding outwards, but that's going to be increasingly far from the center. It's going to be very difficult to build a new airport into a middle of the city (if such needs arises), meanwhile it's much easier to do that for the middle segment of a linear city.