Some of them will let you trick the phone into charging with a full 2/3 amps, even though you've plugged it into a laptop or something that would normally show "charging slowly".

Obviously you're relying on the laptop circuitry to not burn out, but laptops are typically designed with very robust USB circuitry because people often short them out by putting in broken cables.

You might want to keep them around, because sometimes you want to ONLY charge a device, without allowing the possibility of a data connection.

Sure but I always flag them with red electrical tape so their true nature becomes obvious