It allowed them fewer ships than the US and Britain limiting the kind of parity they hoped to have on the world stage in terms of naval power.
One of the arguments went that the US and Britain had to have larger navies because of their need to maintain a force in more than one operating theater while the Japanese only had to worry about their side of the Pacific. It wasn't something that made a number of hardcore military types within the Japanese leadership very happy, but they ended up signing the treaty anyway (though refused to renew it in the 1930s).
It was a water supply for the civilizations within the two rivers. The land was also very fertile and it made it easy to grow food. Lots of vegetation grew there.
