What an offer letter is for
An offer letter tells a candidate that you want to hire them and on what broad terms. It is usually sent after the final interview and before the candidate resigns from their current job, so it needs to be clear enough for them to make that decision.
On its own, an offer letter is not the full employment contract. It becomes an agreement once the candidate accepts it, and the detailed terms are usually set out in the appointment letter they sign on joining.