Why did Sasuke need to activate sharingan to perform chidori?
Very simple, without vision chidori isn't efficient, Sasuke and all users can't hit their opponent, and they're an easy blank for them. That's explaining by Kakashi and Minato in the Manga and Anime. That's the same with the rock, he needs to verify no one is on the road, and verify no one is trying to hit him.
Source: Chidori (Overview - Second Paragraph)