[SunkenCiv]

Could be burials (there’s some sign of that), or could have been some kind of commercial site, and the jars were later reused for burials.

[colorado tanker]

The Plain would be a route from the pre-Khmer territory to Chinese controlled Vietnam, although I would guess most of that trade would go down the Mekong and then by sea.

If I had to make a bet it would be burial with some religious significance. I don't know what kind of beliefs these people had. IIRC, this was a time before the modern Laotians had arrived and this was a little north of the Hindu-influenced area.