Why is a deck a good training task even if you never compete?

It's a fixed, self-checking, repeatable drill that exercises the whole memory stack — PAO fluency, palace stability, encode/decode speed.

* 52 known items in an unknown order — the ideal repeatable, self-checking drill. — Wikimedia user 17177, CC BY-SA 3.0, via Wikimedia Commons. *

A shuffled deck is 52 known items in an unknown order, so it's the perfect practice load: you can shuffle infinitely for fresh reps, you can instantly check your recall against the real deck, and success demands your PAO and your palace and your encoding all work together. Even with no competitive aim, drilling decks is how you turn the individual techniques into a smooth, fast, integrated skill.

Go deeper:

• USA Memory Championship (Wikipedia) — the cards event as a standardised, timed, self-checking drill.
• How to memorize a deck (Art of Manliness) — a beginner-friendly deck drill (uses one-image-per-card, not PAO).