Garen is good for teaching newer players, but mostly from the perspective of the guy who is fighting garen. Fighting against garen teaches newer players valuable concepts like being wary of bushes, being ready to quickly backpedal if the garen suddenly activates his q and wait for him to turn back and use it on a minion, respecting the potential of a flash combo to kill you at low hp before you are able to flash or dash away yourself, ranges of engagement and trading, playing around cooldowns especially ult cooldowns and summoners, how to carefully pressure an opponent who is behind without commiting so that you can keep the regen down without drawing too much jungle aggro, how to bait an enemy into trading with you so that they are defenseless when the jungler comes t gank for you, etc.
Playing garen can teach you things too but it doesn't take a lot of time on garen to start doing things that force other players to adapt. After a relatively low amount of time playing garen you'll have more or less mastered the mechanics involved and will need to learn more about the game as a whole to continue to improve. He still works fine up in mid plat too, especially vs new irelia, so it's not like playing garen means that you're hardstuck bronze or something. Only 10 percent of players reach plat or higher so for almost everyone garen is 100% a viable pick that's not a waste of time to pick up given his low mechanical floor to perform with.