Good Power, STAB and a chance to paralyze.
If you use Milktank as a lead, Stealth Rock is a good option as it can cripple the Pokemon switching in.
This is for Steel Types that Milktank can't really touch.
Wears down the opponent and you can just realax and drink milk
This however does mean you are heavily reliant on Sesemic Toss if you are taunted
Coverage to Milktank's only weakness, fighting. With 90% accuracy it is on the reliable side. It also gives off some flinch chance that can be a lifesaver when you are in a tight spot, unless the Pokemon has Inner Focus or SteadFast.