An offer acceptance letter confirms you are taking the job and restates the terms you agreed: title, salary, start date and location. Restating the terms is the point — it creates a written record that matches your understanding, and any mismatch surfaces now rather than on your first payslip.
Most employers ask for a signed offer letter, but a written acceptance email alongside it creates a clear timestamped record of the terms as you understood them.
Yes. It costs one sentence and catches discrepancies between what was discussed and what was documented, while the offer is still easy to correct.
Practically, no. Negotiate before you accept — once you have accepted in writing your leverage is largely gone.