I don't remember mine saying anything about signing them... now I'm freaking out... . . .
I had a bit of a panic too, but I can't see anywhere in the general guide to completing forms that says this is a requirement. Instructions are different in the Country Specific guides, and for myself (South East Asia & Pacific - 3906) there doesn't appear to be any requirement for this, nor could I see a requirement for this for what I assume is the guide you use for Buenos Aires (Latin America - 3908e). If there is a requirement, it's not obvious, and whether or not it's comforting, you and I at least will be kind of in the same boat. It does seem strange that applicants from the Philippines would be advised to do this though, but it does say to in their guide.
The whole process is rather confusing, but I think it's much the same with immigration anywhere, when my partner came to NZ there were a lot of forms and questions to ask, and lot's of waiting