The benefit is altruistic I would image with a hope of seeking more by that contributor at Pixabay or perhaps elsewhere. The site says, "All pictures are released under Creative Commons CC0 into the public domain." Reward comes through altruism firstly and then there is opportunity.
That opportunity is that someone can 'Like' the image for Facebook or 'Share' it, so it gets further exposure thus the photographer too. Another option is to share coffee with the photographer using their 'Coffee' break link. That leads to a link for a donation through Paypal for any desired amount, although not a requirement.