The main benefit of harvesting rainwater is that the rainwater is usually free from chemicals and falls freely from the sky.
The rainwater is much better for irrigating gardens and keeping vegetables and flowers growing healthy because the chemicals contained in city water can harm vegetable and flower gardens.
It also reduces the amount of the clean drinking water you need to use from your city water supply so it keeps more clean water available for you to drink and saves on your water bill as well.
The water would still get into the ground whether you harvested the rainwater or not so it's not wasting the rainwater to let it go into the ground.
But if you can catch the rainwater before it gets into the ground then it's already there and available for you to use from a rainwater storage tank.
You can even use the rainwater for washing your vehicles which saves the other water for other purposes and also like I mentioned keeps your city water bill lower.