To match the lift distribution over the wing with weight distribution over the wing. Most aircraft have all their weight in the centre (eg. fuselage). The ideal match is actually an eliptical shaped wing planform but tapering is easier to build and is a good compromise.
If a plank flying wing aircraft had its weight evenly distributed over it's entire span then a constant cord would be best. Tapered wing planforms are most often employed to approximate the ideal elliptical planform. Theoretically a perfectly elliptical planform enables every part of the wing to reach the maximum CL at the same time, which will result in a wing with the lowest posible drag for a given lift. Since it is difficult to build a wing with a curved elliptical planform a tapered shape is often used as an approximation.
