I admit this is theoretical only- I have never used one- but I see their use as something that can absorb a cavalry or other heavy charge with little problem, giving other units a chance to counter attack.
A normal cavalry unit would get 9 attacks (charge+terror), of which only 3 hit and 2 would bounce off the save. The steam tank has few attacks, but does have the stand and shoot to help out- getting 1 hit back isn't too unlikely. Cavalry have no support, so I think there's a reasonable chance of the cavalry unit bouncing.
It can either do that, or absorb light missile fire as Honestmistake says.
I think infantry would be its weak point- it can't possibly cause enough casualties to counter the support combat bonuses.