Cross training uses more muscle groups. For example, running builds the lower body. Swimming works the upper body and improves flexibility. Alternating these sports gives great benefits. And, it prevents overuse injuries.
Variety is enjoyable and increases fitness.
A variety of activities can be traded with walking and running. Indoor and outdoor cycling help build strong quadriceps. Competitive soccer builds muscle power and speed. Aquarunning and aquawalking are very good for injured people. Make water workouts feel at least as vigorous as land workouts.