Garen is a great champion with a lot of sustain (passive), he is tanky (W) and a great farmer (E). He is easy to play and hard to kill and thats why i play him full tank. End game you can easily run into all five enemies, dance for 2 seconds and run away without dying.

He is the perfect tank and a great top laner his biggest counters are Teemo, Jayce, Yorick and Darius. Champions you don't see so often especially Jayce and Yorick. But he is great against Riven, Renekton and Nasus some of the strongest top laners of the game.

A really good choice in most ranked games!

You start with Q so you are useful for a early fight it's a great poke and the silence is awesome.

Farm: go into the middle of the minion wave so you hit all six(seven) minions and hit E this will one hit the whole wave mid game. I normally have a farm of 200-300 with garen.

You can see that i'm a big fan of movement speed on garen with boots first item, ghost, and the boots of swiftness - alacrity
This is because you will have to be fast later in the game to chase and escape enemies.
And with randuin's omen they really can get away this is vary useful for teamfights. You will be very mobile with teleport and all your movement speed and able to help your team out at any time

And this is the point you don't care about kills, you don't care if you die (if you can save somebody else) you don't care about anything else then your team and their safety.
You will get enough money for items from farm and you are super tanky because of your W. You are happy about every kill but you don't need them give them to your jungler or mid laner.