Back when I was swimming a lot more than I am now I noticed that there were some benefits that swimming offered running. Strange to say, especially since one includes a jarring amount of gravity against the body while the other leaves the athlete defying gravity and floating along the water.
Here is one of the best benefits I have found that swimming can offer running. This is a positive impact that I have personally encountered in my running that I have gotten from swimming. It might not be there for everyone, but I'm sure there is some sort of benefit for all!
Here is 1 reason why I think swimming helps runners:
Swimming requires breathing control. Running also requires breathing, but you might not have thought of breathing control before in relation to running. As in swimming, a runner should also be able to control their breathing throughout their run. This is likely one of the best benefits for runners I have seen from swimming.
Rhythmically taking in oxygen while swimming without choking to death is a good thing. Being able to take in oxygen without hyperventilating while pushing up a hill, or picking up the pace to pass an opponent and then settling into your natural stride with good efficient breathing rhythm is a great asset to any runner.
Swimming can help you get more efficient in your breathing. In turn, your efficient breathing will make you a stronger runner. Here's how I think this works. When you swim, you breath every 3rd, 4th, 5th, etc.. stroke. As you become a better swimmer, you are able to breath rhythmically after every 3rd, 4th, 5th, etc... stroke while maintaining your speed and strength.
After you begin to master the controlled breathing in the pool, it easy translates to your running. For example, as you are running through a flat portion of land, you might inhale for 3 or 4 steps and then exhale for 3 to 4 steps. This rhythmic breathing can help you control your oxygen intake so that you are more efficiently transferring oxygen to your muscles and not just sucking air.
Now this is certainly not the only benefit that swimming can give a runner. Strength, stamina, injury prevention, and many others come to mind. However, I like this one because you are forced to breath with coordination to be a good swimmer and if you force yourself to breath with coordination as a runner, the benefits could mean a personal record at your next race, or just beating your running partner to the finish line of your next training run.