You can use the site to estimate actual costs for your transaction. The site is not meant for calculating how much a transaction requires gas but it assumes that you already know how much gas a transaction takes.
The Gas used field takes your input and with that input you can estimate how much you actually have to pay for the transaction, given the gas price and gas used. It will also give you estimates about how fast the transaction might get mined.
21000 gas is the minimum gas amount for a transaction - if you only send Ethers to someone you pay 21000 gas but if you include any data (for interacting with a contract for example) the gas amount goes up.
You should read more about gas for example here: What is meant by the term "gas"?