Answer: To cover her with his umbrella on her errands.
When Mr. Bhaer appears to have left town (he has been gone for three days), Jo is first worried and then angry, as she thinks he has left without saying goodbye. However, one evening, she has to do errands in the town, and she ends up walking to the business district, looking for Mr. Bhaer. It starts to rain and Jo's bonnet is getting wet when she bumps into him, who offers to cover her with his umbrella while she finishes her errands.