I have grown up and lived most of my life in the US northeast, in particular New England, more specifically Massachusetts. The phrase "all set" has many meanings which can vary based upon the context. The subtleties are easily lost on most folks, many of whom may be seeing it as a written, not spoken, expression.
It can very well mean you are "ready" (occasionally, one may simply use the word "set" alone):
for an upcoming event
to depart or get going
It also can mean "finished" or "completed":
with the dinner plate, the server can clear it now that you are not using it
the server may offer coffee or dessert but you may want to just get the check and leave
with the tool, you can borrow it without hindering my progress
It can mean "not requiring assistance" or "not requiring (further) merchandise":
just browsing/loitering and not assistance from salesperson/librarian/security/other
after regaining composure, perhaps after an accident, fall, loss of temper
It can mean "the matter is resolved":
shopkeeper: "You are all set." The transaction is complete, the customer can go.
restaurant patron: "This (check) is all set." The money offered for the check will pay in full and the wait staff can keep the change as gratuity, no change required.
repair person: "It's all set now." The item is repaired. (The customer should ask for clarification to make sure that what was requested has been provided.)
As shown above, the many ambiguities for this catch-all phrase depend upon who is speaking and in what circumstance. It is my opinion that because the phrase can be vague, it keeps people communicating. Overly precise language can stifle interaction by crowding out occasion for humorous misunderstanding, actual two-way dialogues where questions and answers are exchanged, and the ability to see anther's point of view by forcing oneself to perceive a situation from a perspective other than their own.
I guess I am all set with this topic.