Last game I went 22/5 and someodd assist as a Garen, I carried...easily, and my build was Force of Nature, Ram Omen, Ninja Tabi, Frozen Mallet, Last Whisper (Which I built after FoN) and was working on Maw of Mal.
The point of building Garen as a tank is simple. If built correctly in a bruiser style, he can put out a great amount of damage while being able to take it. I top lane with him, and to me, the whole point of top lane is to make sure your team can do its job, meaning I have to be ready to bully the ADC and APs out of the fight or into a good picking off position, and possibly initiating or taking some interceptable attacks to protect my team. Building him tanky has great synergy with his W and his passive, but full tank is not the best idea, just like building him like a carry isn't a great idea.
Assuming you are following the meta and topping: You have a ADC bot with a Support, who is usually Sona/Taric/Soraka/Jana/Lulu/Leona of all of those, Taric & Leona are the only non squishes. Further this with your AP mid who is going to be squishy unless the wild mid Diana appears, Leaving the jungle and top to play either tank or bruiser style characters. Now if you build pure damage, you've done two things, your bringing a glass cannon to the table who is susceptible to kiting, and if your jungle is the tank and gotten behind for whatever reason, cannot fill the role Garen is usually used for. Coming out of top lane, I have to be ready to bully everyone, be it a tank, support, or carry.
Can he provide great damage? Absolutely, but why not add some survivability to this as well? Just like the pure tank Garens should add some damage into their build to ensure they can fill their role as well. My suggestion is to build the tanky items that apply the most such as Ram Omen/Thornmail and FoN follow it up with health in any style you want Warmog/Frozen Mallet, and build some damage somewhere in between (Usually I try to go for damage items that offer some tankiness as well just to further the cause.)