Do you mean that there is a plastic bottle of ATF sitting loose under your hood? Or are you asking what the black reservoir with a knurled cap that is full of ATF is for?
If you are asking about the reservoir that is part of the car, it is the power steering fluid. Car parts stores will try to sell you fluid in bottles that say Power Steering Fluid, but your car uses Dextron ATF which is the same thing only it is cheaper when you buy ATF by the quart rather than PS Fluid by the pint.
If you have a bottle of ATF, someone probably put it there so it would be handy when they needed to top off the power steering reservoir. Topping off the power steering reservoir is a routine exercise on many BMWs because their power steering hoses tend to leak.
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