I think they did explain it, although I had to read it a few times to find it. If I understand correctly, they're charging for "the actual binary of the MSI installer for Windows and client specific configuration files." It sounds like they're doing so under the name "GnuPG VS-Desktop." I think it's related to selling services through https://gnupg.com/gnupg-desktop.de.html, but I'm not entirely sure.

They used to ask for donations. They still do, but they used to too. /s