Why do I have to wait before download? [closed]

In some cases, not all, it's so that the site can display some ads for a few seconds more.

In addition to displaying ads, the site uses that time to check the availability of the file at the target URL. In some cases, a referrer is sent so the target knows the source of the "new business", and in some cases, payment is made for the referral.

Sometimes to distinguish between the site's paid service and free one. Most of these sites have a subscription/paid service that should have no wait time or, no ads as well.

There is also another option, when you have multiple location-based services, the file will be downloaded from the closest server to your home. As I know, Sourceforge used to operate this way. You'll get a link, but probably will be a randomly-chosen server.