Simple enough; if it were possible for players to ask a shopkeeper to find and hold an item for him to pick up later, it might help reduce a certain amount of shopping frustration.
Of course, in order to remain balanced, the price will be marked up quite a bit, and the time it takes to find whatever the item is would depend on how valuable it is; asking a shopkeeper to find a nova cannon could take a month or two, for instance.
It'd also be kinda interesting if different stores had different markups for ordering. Relative backwaters would take forever and day to find stuff, and it'd cost a mint as well, while more central locations would be, relatively speaking, cheaper and faster.