They're the American (perhaps other places) equivalent of the little wire ties, almost exclusively used for bread. They're supposed to be quick to close the bread bag back up but in my experience they're I'll equipped for that purpose as they either break or maul the plastic.
I thought they were commonplace until I moved outside the US; at least here in Germany I never see them.
As kids you'd break off one of the half-circle parts, stick it on your finger and flick it to make a makeshift ninja star.
I consider them a disposable freshness seal. After opening a new bag, I toss it out and just spin the bag real tight and tuck it under. The weight of the bread holds the twist.
It’s super effective but my wife isn’t a fan. Neither was my mom.
They're also used where I am with milk to keep the outer bag closed.
At this point, many readers will be regionally thinking, "Milk ... in ... bags???? What's HE talking about?"
But I'm with you, friend. Occlupanida securing the primary containment vessels housing the partitioned units of bovine secretions, all the way!
These used to be absolutely commonplace in the UK with Sunblest in particular, but also supermarket-baked bread, when I was a kid and then they largely disappeared. I haven’t seen one in maybe twenty five years, probably more, but I gather they have crept back into use in places.
Bread bags are pinched and taped, folded and over and taped, or (in the case of supermarket-baked bread) maybe taped paper if not actually simply folded.
Bread in the UK has improved so enormously in my lifetime that we have largely abandoned sci-fi long-life bread anyway, though Sunblest is still around for any quiet men the kids have given cheeky nicknames who are still saving up for a boat. We’ve also largely moved away from giant loaves of bread towards smaller loaves, but the bigger stuff increasingly seems to be sold in waxed paper anyway.
So reusable sealing devices just seem silly.
They were replaced by twist ties in the late 80s / early 90s, which in turn were replaced by tape fastenings fifteen or twenty years later - the main benefit of tape is that it can be recycled along with the bag.
As you say, the industrially-produced sliced white 800g loaf has fallen out of fashion in the UK, and only 20% of us will buy one in any given month. The market is consolidating as a result, with two of the three main providers (ABF and Hovis) in the process of merging.
It's one of those things that we don't really realise about ourselves — our bread is pretty good and our tastes in bread are actually quite grown-up. Kids eat a much, much more varied range of bread now.
And unlike, say, our transition to semi-skimmed milk, it doesn't seem to have really happened as a result of deliberate nudge theory; it's probably more down to cheap flights to the EU and exposure to European bread that people started to remember that our bread used to be varied, rustic and regional, and bakers found that there was demand for pre-Chorleywood breads.
Perhaps it happened simultaneously with our rediscovery of quality cheese.
Including our rejection of margarine, three mass-production uniform-food trends reversed over the same period.
ETA: I guess there was a bit of nudge theory regarding wholemeal bread — was it ever subsidised? Can't remember if margarine alternatives ever had subsidies.