The way I have explained playing Sett this way, these items, these runes, all go back to Sett's release when haymaker only scaled with HP but since the nerfs Haymaker now scales with AD and Health meaning in order to do a lot of damage with Haymaker you need to build AD HP. And since all AD and HP giving items synergize so well with Sett like Goredrinker and Sterak's Gage, which are his 2 best items by far, it's possible to go that build path.

The objective is getting all the health and Attack damage giving items to maximize your W damage and shield. The max damage You can reach like that is almost 1600 which is A LOT that is almost enough to oneshot an ADC or a mage support at once. But the build plays a much bigger role. Your work in a teamfight is to ult a tank in and then stun the entire team, and after that you need to survive and help with the melee champions since after stunning the entire team you W the squishy enemy champions. Now to survive you have A LOT of things to count on. You can use Goredrinker's active to heal yourself and there's also Sterak's passivewhich gives you a big shield and let's you live for a bit longer. And since you survive for so long you can use Haymaker again for another nuke to the enemy team.

Getting ganked as Sett is the best scenario ever since your E stuns if it catches something from both sidesyou can just stun the jungler and kill him and then just run away from your laner opponent. Now ganking with Sett JG is kinda tricky since you don't have a dash and you only have Qs move speed you NEED to have blue smite if you wanna gank really well, either that or the enemy is REALLY overextended.

No one can beat Sett in a level 1 fight with haymaker only very few champions like the threats I mentioned and strong auto attack reset champions that take grasp. Like I said try to rush level 2 and upgrade E instantly and go for a level 2 to level 1 trade. No matter hat happens the enemy CANNOT outtrade you. On Level 3 you can start threatening with your Q by activating it and scaring your opponent and catch a mistake from them. On Level 6 your pressure skyrockets, if you can have a good trade with just Q you can use your ult to bait flash and still catch your opponent since your ult gets you behind them and slows them for 99% which means you can E them into minions for a stun and finish them off with Haymaker and ignite.