Maintaining a fixed Vision point in yoga asana practice is important. Why can be best illustrated with a practical example. If you take any yoga asana (posture) and look at it in detail you can see how the vision point (drushti) affects proper alignment.
For example, the very first asana in the Prana Vashya Yoga ™ sun salutation is Urdhvasana which requires the practitioner to raise the arms whilst looking forward and straightening the spine on the inhalation and then bending back (continuing to look forward) pressing the feet on the floor, on the exhalation. ‘Urdhva’ means upward and ‘asana’ means position. This translates as ‘a position which is held by the upward parts of my body’. If a practitioner looks back when bending back in this asana, then a totally different muscle group is engaged as most of the load/weight of the upper body will go into the small of the back (and injury may even occur). By looking forward the intention of this asana is followed, and the upward aspects of the body are engaged (like the linea alba).
The reason why vision is so important is because it has its own influence on the spine. For example, if I ask you to turn your head to the right, the chances are that you will not just turn your head to the right. Instead you will look right with your eyes leading the movement. So, the eyes tend to turn the head. Put another way: the eyes affect the position of the cervical spine. The cervical spine, in turn, affects the position of the pelvic bone. In short, where you look affects your alignment.
A fixed vision point also assists with balance in postures. More importantly, a fixed vision point helps detach the mind. Looking around the room to see what everyone else is doing is an obvious distraction. When a practitioner focuses on a fixed point there is no information coming from the eyes likely to lead to distraction. This not only helps a practitioner focus on his or her alignment in the posture, but it trains the mind to better focus overall. The eyes lead all the other sense organs. So, control over the eyes through fixed vision points can help with detachment (and therefore control) over all the sense organs (called Pratyahara). Detachment from vision can also teach us to ‘let go’ generally; to let go is to meditate.