Inserting a pattern copies its markup into the post and leaves no record of where it came from, which makes it safe to change and impossible to update.
pattern copied. editing the registration affects
only NEW insertions. no reference exists
in the post at all.
reusable block referenced. a wp_block post. editing it
changes every post using it — and deleting
it silently empties every one.
template applied to NEW posts of a type only.The copy semantics make patterns low-risk to introduce and useless for anything that must be updated centrally. Finding every page using a pattern means searching post content for a distinctive class, which works until the markup changes. Explaining the three options once, by name, saves several confused conversations — the one that surprises people is that deleting a reusable block empties every post referencing it, with no warning and no reference count anywhere in the interface.