This of course happened by accident. How was he to know that his Mangekyou Sharingan even had such an ability. All the same, it had come in useful because at that time, it was the only real thing he could have done. Cause he was sure the older Uchiha could have killed him if All he had was some ninja equipment and some normal jutsu. He would have obviously put up a fight but that man would have over taken him and killed him for sure.
They Did not even fight yet and Sanosuke knew, had it not been for his visual prowess, he did not stand a chance and that was not how he wanted it for himself. He wanted it to be like even if he lost his sight, he would still be feared by all. Just like the previous Uchiha Hokage. As his father told him, the old man sacrificed his eyes during the war and still his enemies hesitated to come to Konoha.