What is the important connection of the document (the Customs of the Tagalog by Juan de Plasencia) to your recognition and appreciation of the Tagalog customs?
The author, Juan de Plasencia was, in the first place, not a native Tagalog but a Franciscan missionary who first arrived in the Philippines in 1577. He was tasked by the King of Spain to document the customs and traditions of the colonized (“natives”) based on, arguably, his own observations and judgments.
It is important to the grand Spanish narrative as civilizer and savior of our doomed souls from eternal hellfire. That's why the Spaniards will quote it forever as their prime authoritative source to show just how primitive we were and how great their contribution was to our development.