Most AC and car-powered USB chargers and USB external battery packs are being available at different current-output levels, typically 500mA, 1A or 2.1A . In most cases, if a charger comes with a particular device, it typically comes with one that is rated to the device’s needs such as a tablet coming with a 2.1A charger.
Connecting a device to a more powerful charger will typically speed up its charging time whereas a less-powerful charger will cause the device to take longer to charge. The classic example is one connecting a smartphone to a high-power 2.1 amp charger that comes with a tablet or an additional 2.1 amp 2-USB charger and finding that this device charges up more quickly than with the charger that comes with the smartphone.
But this kind of connection used to affect older devices which had batteries that couldn’t accept higher charge currents without adverse effects. The recently-designed batteries and device-side charging circuits are now designed to handle higher currents and permit quicker charging. With external battery packs, the amount of power drawn by a device can affect the number of times you can charge the device’s own battery off that pack or the run-time available for your device with that pack if you are using the pack to extend your device’s run-time.
It is also worth noting that if a charger has the standard USB Type-A socket on it, you can use a USB charge/data cable that has the device’s connection (microUSB, Apple legacy 30-pin Dock or Apple Lightning) on it so you don’t need to have chargers for different device types. An increasing number of 2.1A chargers are equipped with two or more USB sockets mainly to allow you to charge two devices at once and the current budget that these chargers put up is shared amongst the devices connected to it. This would typically allow for two smartphones to be charged at a normal rate.
So you can really get by with using higher-powered chargers to charge up your gadgets quickly especially as today’s models are more tolerant of the higher current. Similarly, the use of the 2-USB 2.1A chargers can go a long way with saving on power outlets for charging multiple smartphones.