Both Uses

incidental
in
Harry Potter (#3) and the Prisoner of Azkaban
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Incidentally, that thing you are dreading — it will happen on Friday the sixteenth of October.†
Chpt 6

incidentally = of something that comes with something else, but is less important than it
Right you are, Professor — just giving a bit of background information — the Firebolt, incidentally, has a built-in auto-brake and —†
Chpt 13 *

Definitions:

(1) (incidental as in: an incidental expense)
something that comes with something else, but is less important than it
sometimes in a specialized sense, including:
- incidental expenses or when in context, just incidentals -- minor expenses not budgeted or not specified
- incidental music -- music in a play, television program, radio program, video game or some other form not primarily musical. (The term is less frequently applied to film music, with such music being referred to instead as the film score or soundtrack.)
(2) (meaning too rare to warrant focus)