Requisite Variety in a Nutshell
Simply put, a system needs at least as many control states as it has true states for it to have requisite variety. Beer's example (in Designing Freedom, I think) eludes to a customer entering a department store to purchase an item. If the customer leaves without said item, then the store has lost a sale, potentially to a competitor - the system has failed in its objective of providing an outlet for items to customers.
Increasing Variety
In order to minimise the number of customers this affects, Beer describes several ways of increasing variety in the system, i.e. clearly labeled departments and signposting, directories on each floor, shop assistants wondering around helping customers to find what they are looking for, etc. (#Selfridge's in London at Christmas is a great example!).
Other stuff stores can do is to have information points where folks can go to make inquiries. Carrying a good inventory of stuff in the storeroom is also a good move, as is being able to order stuff into the store - "we are out of stock of that item, but will be getting some in next week so we can reserve one for you…". The whole point being to makes sure that a customer buys something when they are in store (window shoppers excluded).
Impact of Web-based Technology
I was in #Office today looking for shoes. Found the pair I wanted, but they were out of stock in store. The assistant checked the store's online warehouse (using a tablet) and found the shoes. I was able to order them for free next day delivery, to my house.
OK, so I went in for an item but didn't leave the store with it. But the store did not lose my custom, which a year or two ago they perhaps might of done. So by embracing latest technologies, this store has increased its variety.