You can choose to browse through photos by their color. This is helpful when you are trying out a color motif on your IG and want to stick the same colors for creating typography posters.
Never lose track of who the photographer is or that one photo you downloaded a while back and can't seem to find in your files. Download history is not public, so only you can view it when logged into your account (under collections).
Under the "Featured In" section of profiles, you can see the top websites that are using photos from that photographer along with links to the articles/pages.
Photo's can be refused, apparently without any human intervention or review and get a computer generated reply stating that "the quality of the pictures did not meet the guidelines".
Although some images related to your search show up from sponsored web pages (notably shutterstock), they can hardly be labeled as adds. The section is well identified, related to the search, and does not make the page feel too distracting or unclean.
Since pixbay is a free user driven service, it can be difficult to find some pictures related to the subject one is searching on. While in other instances, a flood of results appear because it is a very easy subject to shoot, eg. Road (flood of options), Surprise (debatable options).