“So if he knew when the investigation started they couldn’t charge the President why did they even start it?”
Because he was charged with investigating possible Russian interference and possible collusion. He found Russian interference, he didn’t find collusion.
Possible obstruction came up during the investigation, but it can’t be charged in this situation, and he said that too.
We need to see the scope memo,
IMO they knew there was no way they could charge Trump so instead they kept the charade going merely to keep a cloud over POTUS, with the intention of affecting the midterm and 2020 elections.