Briefly what the $THING is should always come first (in everything you say and write, formally and informally), so you can decide whether it's relevant to you. If it is, then you can read the motivation and historical significance.

I thought the second section did a much better job explaining what the pattern was and what the advantages of it are! The first section tried to set up the background for PRPL, but these justifications don't belong in the user-facing documentation (except as an appendix).