Why have a default user password feauture?

Hi,

  I (understood) read in the manual that there could be a default user password -

28 BF 4E 5E 4E 75 8A 41 64 00 4E 56 FF FA 01 08
2E 2E 00 B6 D0 68 3E 80 2F 0C A9 FE 64 53 69 7A.

If the file is encrypted based one this key, readers can open the file without having to prompt for password though the file is encrypted. I want to know whats the point of this? It provides no security. All it ensures is the reader be sophisticated enough to have a security handler. I don't see the point. Is there any advantage of having such a pdf with default user password?.What was the motivation behind this?

Thanks