It helps to know how to cross-train
Even if you consider yourself solely a runner, it’s still beneficial to know how to cross-train.
At some point, your fatigue will outpace your desire to train. You’ll want to train, but your legs will say no.
If you knew how to cross-train on the bike or by swimming, then you’d be able to harness that motivation to train. You’d be able to gain that much-desired aerobic benefit, even with your legs feeling exceptionally tired.
At some point, you may also feel a niggle. If running is your only outlet, then you’ll be tempted to continue training despite this niggle. This will lead you to injury.
But if you had a cross-training option, then you’d find it easier to respect the injury risk. Swimming gives you the option to train while respecting the signals your body is sending.
The cross-training might feel painfully awkward at first. You don’t have the repetitions, so you can’t expect to be good. But you’ll quickly pick up the skills. And you’ll be better positioned for the future because of it.