Now that you mention it is said when somebody's remark reminds you of something. Now that you mention it, it did smell of gas.
It gives the other person credit for the thought, which is what makes it courteous. It usually introduces agreement that had not occurred to the speaker before.
1phrasenow that you mention itSaid when somebody's remark reminds you of something.
“Now that you mention it, she did leave early.”
now that you mention it is a phrase.
now you mention it Shortening of now that you mention it
Etymology from Wiktionary, CC BY-SA
Practised in Small Talk & Everyday Phrases