Let's say that you want to insert a call to action (CTA) in your posts. You could use either a block pattern or a reusable block depending on your goals.
are more flexible.
Once you insert a block pattern, you can fully customize it with your own content to make it unique. Any changes that you make to the block pattern after inserting it only apply to that piece of content.
If you plan to personalize the CTA's content for each post, you might want to use a Block Pattern.
The block pattern will eliminate the need to recreate the basic design for each instance, but you're free to update the content as needed without changing the other instances of the CTA.
If you want the CTA to be exactly the same in every single post, you'd want to use a reusable block. If you ever want to change the CTA in the future, you just need to edit the reusable block one time and all instances of that CTA will automatically update.