It hides what you're searching for, ignores your distance filters, and fills your results with items that are essentially ebay cross-listings. The pictures are extremely compressed so you're unable to validate the item from pictures before reaching out.
The only benefit is many people already have a facebook account, but now I'm unable to delete my facebook account because marketplace is the predominant classified sales channel in most areas. It was a EEE style effort to collect more data on people.
The past few years there have been persistent spam campaigns against Craigslist in various areas of the for-sale classifieds. It wouldn't surprise me if meta was behind it.
Groups are in a similar vein. They've put a log-in barrier on various communities, lowering the barrier to entry tech-wise, but forcing people into their ecosystem when forums have existed for years.
These are bad services. They gain traction due to facebook's other malicious actions.