Generally speaking, the intention is usually to act as a compromise between having limited restocks versus being readily available with no restrictions. It effectively limits how many will be in circulation by a significant factor, while still allowing for everyone to get one and not be frustrated by restocks to even get that one. This became more common as a method after we stopped doing restocks for most items, so that some items could still maintain a greater sense of rarity. There are some other approaches that could work but they're not currently as easily technically feasible.