Dr. Yang Jwing Ming, in his book “Advanced Tai Chi II” reveals there is much more to the Tai Chi fighting set than appears on the surface. While resembling a dance like form between two people, at another level it becomes a very advanced sort of duel between both.
Where one side is neutralizing each attack throughout the set, if either person over commits at any point in the form, either too much power or a movement where they are improperly unaligned, then the partner can neutralize the attacking movement. Dr. Yang describes in great detail how this is done. This makes the practice become a duel between the parties, sort of an advanced pushing hands practice.
Used thus, you are working on recognizing the moment during an attack where the attacker you face is open to a counter. At the same time they are looking to exploit your mistakes.
This makes this study most valuable. It is so in all the variations of Chinese two person forms, IMO..