syntax to update the value stored under existing atom keys

It’s interesting the documentation calls out atom keys specifically. You can also use it to update non-atom keys (though the syntax is slightly different, I suppose):

iex(1)> my_map = %{"a" => 3, "b" => 4}
%{"a" => 3, "b" => 4}
iex(2)> %{my_map | "b" => 5}
%{"a" => 3, "b" => 5}

Shortcut syntax is particularly concise when dealing with structs since they fulfil the two preconditions definitively: (a) atom keys and (b) keys are known to exist.

They are less useful with dealing with arbitrary maps because one or both of the preconditions for using shortcut syntax can’t be assumed.