A block pattern is a template — inserting it copies the blocks into the document, and edits do not affect the pattern. A reusable block (synced pattern in 6.3+) shares one instance across every insertion.
Use patterns for layouts you want editors to customise per page. Use synced patterns for content that must stay identical everywhere — a CTA banner, a header bar.
The 6.3+ pattern manager treats both as one concept with a sync toggle, which is a much clearer mental model.