Leona's all-in style of gameplay heavily revolves around jumping on your target and killing them as fast as possible. Aftershock's explosion damage further pushes your ability to kill the enemy after CCing them.
Demolish allows you to snowball early lane kills into massive tower damage for an early team gold lead.
Conditioning on Leona is great once you've hit the 10min mark as both the bonus Armor and MR contribute to your [W] strength.
Unflinching's slow and cc resist empower your ability to stick to your target after engaging onto them for follow-up CC and damage.
Hextech Flashtraption is great for those moments when you are able to hide in fog of war on a pink ward and jump your opponent with a well timed all-in engage.
Cosmic Insight's well-rounded utility empowers your ability to chain CC the enemy and peel for your carry.