You use incidentally to introduce a point which is not directly relevant to what you are saying, often a question or extra information that you have just thought of. `I didn't ask you to come. Incidentally, why have you come?'
If something occurs only incidentally, it is less important than another thing or is not a major part of it. The letter mentioned my great-aunt and uncle only incidentally
in an incidental manner; "these magnificent achievements were only incidentally influenced by Oriental models