The rated current is the maximum. The device - a phone, a tablet, a USB light, whatever - takes as much as it is rated or less. In other words, the real current will be the smaller of the two: the maximum rating of the power source or the maximum drain of the load. If the source says it can provide 8A, it does not mean it will push 8A to a device that can take only 2A; it will provide only as much as the device takes. I hope I made it clear enough.