For strength or for combat?

Different schools teach tai chi chuan in different ways, and the purpose of the form changes relative to each school's agenda.
Low stances and long stances are commonplace.
These postures indicate that the form is being used to develop strength, and that combat skills are trained separately (if at all).

Master Waller's form is practiced for combat. The postures are exactly the same as the self defence movements. 
Strength and combat are trained simultaneously.