Advertising, to pay for the free service.
To the right of the
Downloading in.... message you will also find
Mirror Provided by: and
Scanned by:, while I doubt these are paid ads, it would be in return for providing the mirror/scan service, underneath you will find paid ads under the
You may like:. Then under that, you have other top projects in the same category so that you may keep looking and download something else, to get more ads.
By making you keep the page open for a few seconds, they have time to display ads that might get your attention and increase the chance of revenue from you clicking an ad.