The importance of prehab prior to ACL reconstruction is simply to optimize your ultimate post operative outcome. The better your motion and strength before surgery, the smoother the recovery process will go!
After sustaining an ACL tear, you will likely experience swelling, decreased motion and quad inhibition (decreased activation of the quadriceps muscle).
The single most important item to conquer is achieving full knee extension/straightening before going to the OR. Formal prehab with a sports medicine physical therapist will also work on controlling joint swelling, regaining knee flexion and normalizing gait.
The use of a brace and crutches when out and about in the community will reduce fall risk; as a fall could result in further injury to the meniscus and cartilage. An emphasis on each of these things will better set you up for success following ACL reconstruction, and help you achieve your long-term fitness and sport goals.