"Post script is usually written when a writer forgets to write something on the main body of the letter and wants to add on to what he had written on the letter." But in this age of email and word processors, why would anyone want to use P.S.? Is its usage still relevant and any particular reason for it?
I usually use it purposely to add something related to the email, but that I don't want to mix with the rest, likely to let the reader to focus more easily on the part of the email I think it's more important; I like to use it like a sort of "side note", placed after the written text (lit. latin "post scriptum" can be translated as "after the written (text)")
The postscript is indeed of limited use, but it might still be useful when one has something else to say, but doesn't want to compose the email all over again.
People still do compose letters from time to time, and send them on paper. I do it maybe once a year, and I can see it coming up where I have something to add, but don't want to print the letter again.
There may well come a time where people don't know what "P.S." means, just as some people don't realize that "R.S.V.P." means to reply soonest, please.
P.S., It also might be useful if one is mimicking an old-style letter format.
I would not include a post script in the body of an email because post means afterwards, so if inserted into the body, it's no longer "post". As one responder previously mentioned, a post script is an added thought, often of some short comment entirely different, or as a brief comment that contributes to what has been said, but gives a short insight or some other contribution to what has been written. I use post scripts in hand-written letters frequently, but since emails are generally shorter and more succinct, I have less need to use them in that format. I find that with emails, I insert additional thoughts in parenthesis or in between dashes just as in letter writing, but because of the shorter nature of my emails, I rarely use post scripts.