The importance of this point in Tai Chi practice is multi-faceted in terms of a postural and balance guide, the idea is that when the weight falls properly on this point one has aligned the weight of the upper body correctly in respect to the base of the lower body. The feeling of this correct alignment is that the foot, even of the weight bearing leg, should be soft and relaxed. It is interesting to note that most of your muscular usage (some tests say as much as 80%) is compensating for poor balance. So as our balance and posture improve, we become more efficient in our muscular use, not only conserving energy but also freeing the body to move. This is a prime contributor to the strength element in Tai Chi. The awareness of the desired feeling of the foot being soft and relaxed is one of the most important indicators of this correct body relationship.
