You use incidentally to introduce a point that 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?'
“我没有叫你来。顺便问一下,你为什么来呢?”
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.
信里只是附带地提到我的叔祖母和叔祖父。