The service has offered free downloads since past many years but new changes by Youtube requires processing of Full-HD and MP3 downloads on servers which attracts expensive infrastructure and bandwidth costs that needs to be paid to keep this "easiest" method of downloading videos alive.
While the server costs are recurring we are only asking for a donation, new donations keeps things running. This upgrade not only allows us to keep this service alive, but also offers you to enjoy a much FASTER, cleaner, ad-free and streamlined downloading experience (as shown in animation below) you will not find anywhere else.