(I was thinking acquisition would be when I paid for it and inventory would be when it actually arrived in my hot little hands.)
That's exactly how I use those dates. For Kickstarter campaigns, I fill in the date the campaign closed as Acquisition Date. When I buy a game at my local store, I fill in both dates to be the same.
I don't think there are any functions on BGG that use those dates though. It might be that Friendless' extended stats make use of them in some way, but that would be assuming those fields are even retrievable through the API.
Link to my Friendless' extended stats page: http://friendlessstats.dtdns.net/dynamic/tabbed/SpecularRain