In addition to the response on wallet.dat. IF for example you import key1/key2 from one computer to load the wallet there, you will have to hit the rescan in that scenarion as well.
So what does "rescan do?"
It simply scans the blockchain for all of your inputs(money) and sees which ones it can spend. The only purpose for wallet.dat is to store that data.
So if you install on a new computer as well wallet.dat will be blank and if it synchronizes it did so not looking for the inputs. It needs to "watch" the blockchain.
Thus, I use a very little known version of blackcoind/bitcoind/bitbayd called "watchonly" which allows for the ability to see what is available for accounts that are not yours or multisignature accounts that you dont have both private keys for.
This also means you can have key1 on one computer/device and key2 on a second computer/device and perform sending in two steps.
Yet another way to make your account completely hack proof.
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