There are several possibilities. One is that the merchant who gave her the flyer asked to collect the mobile phone numbers of people who may become customers. Later, they may call you to promote products... It is basically a task, so this Don't overthink this situation.

The other thing is, that girl is interested in you, maybe she thinks you are quite handsome, and wants to have some contact with you, so she adds you on WeChat or something. If you have some feelings for her, why not Just take the initiative to learn more about it.