Here is a basic summary of the economic sense behind floating storage:

http://oil-price.net/en/articles/the-case-for-floating-oil-s...

Some places have things like this http://en.wikipedia.org/wiki/Louisiana_Offshore_Oil_Port that means the tanker doesn't have to come to a traditional port.

Often tankers sit out and wait for the oil price to change marginally (yes, really - its that silly). Other than that, could be many reasons including bunkering (via smaller ships), full port, repairs/servicing, etc.

It's not that silly when you work it out. You have a ship holding 2-3 million barrels of oil and a 20-40 crew. It looks like the market price of light sweet crude varies by as much as $1 per barrel per day. Which means a difference of 2-3 million dollars if you come to port today instead of yesterday.

Not silly in that sense. Silly in the pragmatic sense that expensive ships spend considerable amounts of time doing nothing...