Bitcoin sent to your old addresses will still be received in your wallet, however, to protect your privacy new addresses are generated automatically once you use them. Doing so allows you to isolate each of your transactions in such a way that it is not possible to associate them all together. When addresses are re-used, it allow others to much more easily and reliably determine that the address being reused is yours. All bitcoin transactions are public, traceable, and permanently stored in the Bitcoin network. For this reason, we deem it best practice to ensure Bitcoin addresses are only used once.