They display better on mobile devices:
Think about mobile users who fill out forms. When a user selects a form field, the mobile web browser automatically zooms in on it to make it easier for users to see what they’re typing in the field. Zooming in on form fields with left aligned labels makes the label disappear off the user’s screen. Users have to horizontally scroll to see the label and scroll back again to fill out the field. And they have to do this for every form field. This is a painful experience that users shouldn’t have to go through.
Read some more about form design and then check your your online library card application.