It is way to let others know that you have already expressed an interest and so that there is no bickering about who asked first.
That makes more sense - sorry for jumping on you.
I didn't consider that because PMs are opaque to the public, and as such the seller can simply pick and choose who/what to disclose, and even outright make up a "first" buyer - and so I wouldn't weight public disclosures of PMing highly - but I suppose everyone is different. Trust and integrity aren't known quantities, and certainly not in the anonymous/informal/casual digital age, full of moral hazard.
Best,
Tal
