Firstly, an offer letter is relatively binding, but only on one side. Rescinding a documented offer will create a PR disaster, if not a legal default fromfor the company's side. HoweverYet freshers routinely disregard the sanctity of an accepted offer. So if the offer letter has not been issued, plan changes are simpler.
Secondly, ifthe moment HR gives a fresher an offer letter, the fresher will use it to shop around and get another higher-paying offer. In the eyes of HR, having an offer letter that the fresher can show to other companies itself reduces the probability that the fresher will show up on the joining date. No offer letter equals fewer no-shows.