Isn't it pointless to care about GDP? If I pick up a stone there and sell it to you for 1 million dollars, 1 million dollars added value is created and added to GDP. If you sell it to me for the same price, virtually nothing has happened but GDP increased by 1 million dollars.

We can increase GDP by any number we want.

Why do we care about GDP?
Yes, GDP can indeed be inflated by meaningless circular transactions. Furthermore, GDP growth completely ignores underlying changes to a country's asset base. Corporate accounts are considerably less naive. Any company that only ever reported growth in turnover wouldn't be trusted: turnover is vanity, profit is sanity.

Real increases in GDP can be a useful proxy for economic growth, because it measures the total transaction value within the economy, and historically that's been closely correlated to general perceptions of economic growth. So much so, that it's now pretty much used as the definition of economic growth.