This build works because it increases your chances of hooking an enemy in lane. Blitzcrank is regarded as a weak support because of his weak laning phase and his hook is usually easily played around by good players, but smite makes sure that these players must respect your smite q combo. You can even leash for your jungler by taking 2-3 hits from their first buff and smiting to regain lost health. By the time the 80s cooldown on your first smite is finished, you will have an opportunity to hook smite, hit then e to deal your full combo. Do not be surprised if you see this build in the LCS anytime soon, but please do remember that this build originated from a lowly gold player.