What makes people give their time but not their money?

Dear friends,
The other day, while walking in the tourist center of an European town, I saw something that I had seen millions of times: a couple asking a passer-by, who was wearing a suit and walking hurriedly towards his home, to stop and take a picture of them. This common request somehow made me think about the following: why did the passer-by stop? Had the couple asked him 5 cents he would have never stopped, but asking him his precious time he agreed.
What makes people give their time but not their money?