Original Message

"Why is this necessary to do autos ????" 

Power failures and some other issues recommend an autorotation.

I will frickin' A guarantee you that your first power failure and autorotation will not be like those done in training. Your first indication may be a yaw (got my undivided intense attention) a caution panel indication/low NR horn. You will need training to minimize the analysis and response in as timely a fashion as possible. 

Your training should teach you how about NR, airspeed effects on the autorotational 'glide' and how those factor are used in the decel and landing.... ahh, the landing! Autorotation proficiency markedly increases your chances of not damaging or minimal damage to the aircraft in whatever your landing situation. 

Decades ago when I was teaching and when most Army aviators were proficient in autos, I read statistics that showed about 90% of the Army's emergency autos were successful- day, night, IMC.

Good training means good flying.