This is a unique build. Many user see Garen as nothing more than an AD champion who is able to dish damage and leave. I Say they can GTFO and use Garen as an AD tank and let his blade do the talking. This build will allow you to Apply a lot of damage and leave with a good bit of your life bar still in tact. I have gone into battle a lot with this build and came out saying to myself "I have too much Health for this ****.". Also this build allows you to save your team and get in front of your opponent when a friendly team champion is being attacked.

Pros

Long gank survival
Excellent 1v1 kill time
easy escapes if needed
Large amounts of gold earned easily in laning

Cons
Usually targeted for multiple ganks
not well against large amounts of armor build

Flash and Ignite are useful for this build because while he is quick there are always possible ways a champion can escape. so flash is for the times when they are escaping with near large or half health and you need to attack more or you just want to harass. Ignite on the other hand i consider a DPS nuke. when a champions health is running low you hit them with ignite on their way to escape and pick up an easy champion kill.

This skill sequence simply put is more towards dealing easy damage while also providing a great way to survive. You want to have judgement upgraded first because during a early and mid game experience, farming is an important factor so judgement with your high cooldown reduction rune build is a great way to pick up quick minion kills while also escaping from champions trying to chase you.
With decisive strike the main goal is to have a way to chase down a champion and deliver that final blow when you need it most. With it's burst of speed and damage output you get a powerful strike, and since it silences the enemy champion for a short time you cam get those few extra hits in with the extra attack speed Garen has acquired.
Simply courage you want to have fully upgraded last because while it does increase defense and magic resistance, You want to have that damage output and escape techniques handy and more powerful earlier in the game when they are most useful and champions have less health. Since courage also applies cooldown reduction, since Garen has no mana he becomes and immediate ability spammer and a near carry player.