You do if you want to get the vehicle back.
If they repossessed the vehicle, it's usually because you defaulted on the loan. Even if you tried to pay them after the repo, chances are, they would refund your payment. The laws vary state to state, but there is usually a redemption period after they repo your vehicle. The redemption period is the time between the date of the repo and the date they auction or resell the vehicle. You can buy the car back, but paying arrears may not be enough. You may need to pay the remaining balance of the loan in full.