The flare at the end of an autorotation is to arrest the decent.
When in an autorotation, the decent rate is significant, upwards of 1,500 feet per minute. This rate of decent must be reduced before attempting a landing. The flare reduces the decent rate. As a side benefit, the flare also increases the rotor RPM and reduces forward speed.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 11-4