There are more than two reasons a user might upvote another user's post.

Some of the many situations one might click the up arrow:

- to thank others for posting to one's thread in order to create a positive feedback loop

- to increase visibility of what a user wrote or submitted

- to display agreement with points made in the submission

- to "save" the submission for future reference, as users can track what they've upvoted in the past

- to promote a specific idea or ideology