He's a monstrous protector and a strong laner.
- His passive provides a massive amount of sustain during teamfights so long as you keep autoattacking something.
- His Q and W provide reliable CC and let him avoid or interrupt enemy attacks.
- His E is a strong vision tool, useful for scouting or guarding areas.
- His ultimate reduces the damage taken a percentage, letting them survive far more damage simply for being near him.
Basically, he's extremely good at protecting his team from enemies and enabling them to survive and deal their damage. His lockdown is very reliable and his utility (from vision and damage reduction) massive.
In addition, Maokai is a relatively strong laner and rather safe against most match-ups. His WQ are strong tools for assisting ganks or when performing teleport-ganks, which make him a great match with damage-oriented junglers (i.e. Elise, Lee Sin, Kindred, etc).