Importance of being strong for endurance activities.
The stronger you are the easier it is to perform a hard task.
The easier the task is the less effort you need to perform the task.
The less effort you need to perform the task the longer you can preform the task for.
Running = the stronger you get your lower body the easier it is to produce the same ground force as before.
Swimming = the stronger your upper body the easy it is to apply you previous amount of force to the water.
Increase efficiency = decrease in energy consumption