used before saying something that is not as important as the main subject of conversation, but is connected to it in some way

We had a marvellous meal at that restaurant you recommended - incidentally, I must give you the number of a similar one I know. 我们在你推荐的那家餐馆美餐了一顿——顺便，我一定要把我知道的类似的一家餐馆的电话号码给你。

used when mentioning a subject that has not been discussed before, often making it seem less important than it really is

Incidentally, I wanted to have a word with you about your travel expenses. 顺便说一句，我打算和你谈一下你的开支报销问题。