Why do doctors recommended using minoxidil (5%) for 6 months for newly transplanted hair?

Minoxidil is a non-specific medication that promotes increased nutrition and bloodflow to the roots of the hair. So the benefits are good for newly transplanted grafts.  Minoxidil is not necessary though. Your transplant will grow in fine.

Minoxidil is effective in protecting the non-transplanted hairs by keeping them in anagen phase for longer.  Patients often times need a blueprint for maintaining native hairs so that future procedures are not necessary.  Transplanted hairs have been reported to grow faster with minoxidil.  Evidence has been reported by several studies with limited numbers including at a Consensus Report at the 9th Annual Meeting in 2002.  The main downside of minoxidil is irritation which is often mitigated by the use of foam.  Best Dr. Anil Shah

Personally I don’t recommend minoxidil (5%) after hair transplant, because the hair can fall down really quick after stop the medicine.

Minoxidil is an additional treatment that can be helpful to stimulate hair growth both from the newly transplanted hairs and from the existing pre-transplant hairs.  While it can be helpful, it is not required pre or post-transplant.  Lack of minoxidil use will definitely will not cause the transplanted hair to fail or to fall out.  
All the best,

Not all doctors recommend topical minoxidil after hair transplant surgery.  If your doctor recommends it, then you should follow your doctor's advice.  If you do use it, and you get great results you need to continue using it to maintain the results.

Minoxidil after hair transplants is used by some surgeons. Over 35 years of doing grafting I have seen no proof that there is a benefit and can cause hair loss in the raining hair when stopped. 

Minoxidil is thought to allow the transplanted hair to grow faster, but this has not been tested in well-designed studies. Some hair transplant surgeons also report lees shedding of the transplanted post-op with minoxidil.Whichever the case, it is not necessary to use minoxidil after your hair transplant.
Topical minoxidil is the mainstay treatment for androgenetic alopecia, helps your existing hair from falling out and only works as long as it is used. The transplanted hair is genetically resistant to male pattern baldness and does not need minoxidil.

Finasteride and Minoxidil are usually prescribed for 6 months following hair transplant to prevent shock loss and prevent non transplanted hair from thinning and falling. As male androgenetic alopecia is a progressive condition, non transplanted hair in the anterior, mid scalp and vertex will continue to be under the influence of DHT and tend to miniaturize and fall. Hence to prevent further hair loss, one needs to be on a cyclical regimen of Finasteride and minoxidil for few months in a year.

Minoxidil is primarily used for preventing shock loss of existing hair ( if hair have been transplanted in between) or to stop hair fall of the existing non transplanted hair.The new transplanted hair will go even if you do not use minoxidil.

Rogaine can be used after hair transplant to minimize to:Minimize the risk of shock loss after hair transplantMaintain the rest of your hair and prevent further hair lossIf you are not using Rogaine or Propecia after your hair transplant you may experience further hair loss in the areas that are still prone to hair loss. You need to check with your doctor and see what is recommended based on your levels of active hair loss.