There are many important reasons to learn to swim.
Water Safety: Drowning is the leading cause of unintentional death in children under 4 (https://www.poolsafely.gov/). Swimming lessons are an essential element of drowning prevention.
Child Development: Research has shown that swim instruction not only prevents drowning, but also accelerates child development. The Griffith Study, which looked at 7,000 kids in swim lessons over a four-year period, determined that children who were enrolled in swim lessons developed several months ahead of their peers. They performed, on average, 11 months ahead in oral expression, six months ahead in mathematic reasoning, 17 months ahead in story recall, and 20 months ahead in understanding directions. (Griffith University)