I booked a domestic hotel. After confirmation, HRS sent me a confirmation letter of hotel reservation. What's the use of this?

You have to confirm your reservation with the hotel first (when you confirm, tell the hotel that you made the reservation through HRS, and then tell you the name you used when you made the reservation and the type of room you reserved). If the hotel says it can't find it, you can call HRS to ask for confirmation, because you have a hotel reservation confirmation letter, HRS will be responsible and must arrange a room for you.