Makes it fair so everybody can get some rather than a few getting it all, common sense really, you are being rationed, means not a dew hogging it all.
The choice of words in the patch notes is a bit miss-leading. The wording would suggest that previously the supply of rares at a station behaved like normal items, in that everyone at that station had access to the same stock level and if one person bought it all, no-one else could. That was the initial way that rares worked, but it was changed in gamma so that you have a supply amount that was unique to each person.
Before the latest patch, if a station showed a supply of 15, then that stock was purely for you and no-one else could access it. You could wait for re-stock ticks, every 10 minutes, and slowly fill up your cargo bay over however long it took to fill - in some cases it took several hours.
This patch, although worded like the gamma patch, has actually limited the total amount that a station will sell you. Once you buy the amount (eg 10 tonnes seems to be the limit of Bast Snake Gin, I have found after some buy/sell experiments) the station won't sell you anymore. If doesn't matter how long you wait, you're capped out. It's basically a "10 items per customer" situation. If you sell the items back to the station, on the next tick the supply goes up, but it will never sell you more than a total of 10 (for Bast at least). You have to sell your stock before you can buy some more.
Rares used to be an alternative to normal trades in that you made one really long trade-route instead of multiple small routes. The time it took to fill your hold plus travel 150+ly got you a vaguely similar profit/hour as normal trading (early gamma rare trading aside). It was just a different usage of your time and effort.
