Why do they always have to announce they are leaving?
I have no idea unless they want a parting gift.
Two good reasons come to mind: One, so concerned FRiends won't incorrectly think that ill health or ill fortune has interrupted their presence here. The principle is that of asking to be excused at the dinner table, dispensed with in less formal circumstances [a sandwich at the kitchen table] but maintained for more formal occasions. I hope you're familiar with such formalities, and appreciate their presence here as elsewhere.
The second reason: that we miught note the missed presence of one of our own, and hope for an eventual return:
And he spake this parable unto them, saying,--Luke 15: 3-7 (King James Version)
"What man of you, having an hundred sheep, if he lose one of them, doth not leave the ninety and nine in the wilderness, and go after that which is lost, until he find it?"
And when he hath found it, he layeth it on his shoulders, rejoicing.
And when he cometh home, he calleth together his friends and neighbours, saying unto them, Rejoice with me; for I have found my sheep which was lost.