You give the offer a real chance to be seen
You used to swap out the offer every post, so it disappeared before it reached anyone who hadn't seen it the first time. With the habit of keeping the same focus offer across several appearances, more people who hadn't seen the product or the promotion end up seeing it, instead of the shop restarting its reach from zero with every post.
You spend less time thinking up a new offer every week
You used to pick a different product or promotion for every post, eating up time that could go to serving a customer or restocking shelves. With one focus offer set for a longer stretch, the work shifts to varying the format or the moment of that same offer, not inventing a new one every time it's time to post.
You make it clear to customers what the offer of the moment is
When you changed the offer with every post, followers didn't know which product or promotion was actually the priority for that stretch. With the same offer showing up in different ways, the customer understands more easily what's being offered, instead of getting confused about which promotion came before.
You can tell if the offer is working or not
If you only showed an offer once, you didn't have time to notice whether there was interest or not. Repeating the same offer in different formats or moments makes it easier to compare what happened from one appearance to the next, and decide with more grounds whether that offer is still worth keeping.
You see clearly if you're repeating with a plan or just posting
Before this session, you couldn't say whether you were repeating an offer on purpose or just reusing a post with no other idea. After identifying what changes between one appearance and the next, format or moment, that distinction becomes clear when deciding the next post.