She's meant to be a tanky, high damage support. That's the whole point of her sunlight buff along with having 4 damaging abilities.
She's also lived in Thresh's and Braum's shadow for far too long. Thresh has great peel, great utility, and alright engage. Braum has amazing peel and good engage. Leona is being solidified as a powerful, engage as her kit is designed all around it. However, with engage kits, they typically come with damage.
It's also a nice way to make a kit more fun. Gives strategic purpose to her W now. Hit as many targets as possible instead of mindlessly swinging at the carry to get the bonus duration just for hitting something. The better your positioning/worse their's, the more you're rewarded.
Plus, we might see some jungle Leona now. I don't remember if the spell specifies champs, but if champs and monsters, could def see some jg Leona.