When you swim, bike and run, you're basically moving forward in a straight line the majority of the time. Sure, you may turn right or left, but you probably don't incorporate lateral movements into your workouts very often.
Muscle imbalances can start to build in your body when your routine is stagnant, and injuries can start to creep in due to the overuse of the same muscle groups.
Spending 10 minutes, twice a week doing drills that work those muscle weaknesses can improve balance and reduce the risk of injury. These drills and exercises are great to use as a warm up before you start your strength training session, or they can be used to wrap up an easy day on the bike or in the pool.