Mostly collectibility at this point although there are a lot out there of unreleased mixes or masterings that never made it out into the world for one reason or another. There are a bunch of VMP ones for unreleased albums (Nilsson Sings Lee Newman, Lauryn Hill MTV unplugged I think, and more). Those won’t be in the sale. They did say they typically got 3 test pressings for most albums unless it’s something like Shabaka where the TP was rejected by the artist then they recut it.
I think in the modern day and age, test pressings aren’t likely to sound much better than the general release and they often don’t include the album sleeve. Back in the day, many records probably got much bigger pressings so the test pressing likely did sound better than the 100,000th record off the presses from worn stampers.
I’m with
@TenderLovingKiller® that white label promos are pretty rad.