You could create an address and use it a bunch, it will work. However it starts to get hard to keep track of all the money when they keep going through the same address. There is also a privacy issue. Suppose I pay you with that address. Like anyone, I can check the balance of that "addy" at blockexplorer.com or elsewhere. If you use a new addy each time then no one will know your balance because it will be spread across a bunch of seemingly unrelated addresses. A business might value this more than you. And some businesses go with one addy per customer, for example. For an individual it may not matter much, but it does help with book keeping. Does that help?