A conversational phrase meaning 'while you're at it' or 'since we're already doing X, we might as well do Y'; used to add an action opportunistically.
ついでだから、スーパーに寄って牛乳を買ってきてくれる？
While you're at it, could you stop by the supermarket and buy milk?
図書館に行くついでだから、返却ポストに本を入れておこう。
Since I'm going to the library, I'll drop the books in the return box.
ついでだから、その資料もコピーしておいてください。
While you're at it, please make copies of that document as well.