People layer for fashion purposes, or to be warm, but you mentioned it being summer time. As someone said above, sometimes it is actually a single shirt to look like two. Some parts of the US, even in the summer can be cool, especially at night. Some restaurants and shopping malls are over air conditioned and so people where long sleeves even when it is warm out. Also, I cover my arms at dusk and night time to avoid mosquito bites in the summer, or to protect my arms from the sun. Depends on the situation.