The reason that these 10 days are so massive and so rewarding is in them; We have day of arafah like in Ramadan we have laitual Qadr. The messenger of Allah sallallahu alayhi wa sallam, He summarized the whole Hajj and he said Hajj is Arafah.
The messenger of Allah sallallahu alayhi wa sallam said the day that is promised is the day qiyamah and the day that is witnessed is the day of Arafah and the day is Jumah. And the day of Arafah there is one hour. If this hour passes by a mumin that is asking Allah subhanahu wa’ta’ala for khair (good), Allah would answer the dua of that person.
This hour comes to a servant of Allah while he is asking Allah to protect him from a certain evil then Allah will protect that from him. If you are outside of the hajj and you fast the day of Arafah, then Allah will forgive two years of your sins.