Those 12V drivers are designed for driving LEDs off of 12V low voltage halogen circuits. Most are for replacing MR16 style light bulbs. They work great for making lights that run off of car power or wall warts. Almost all of them have input bridge rectifiers and a filter cap. They can run off of AC or DC power. Those that use the PT4115 chip can work with like 30V in (and the chip has a rather versatile dimmer pin). I used one in my desk lamp retrofit: Nichia 219 Desk Lamp Retrofit
I also use them for testing/driving various 1 to 10 watt LEDs. I usually just glue the LED to a small heatsink and attach a 1W/3W/10W driver. I have around a couple dozen test rigs set up that way. The drivers are cheap enough to not mess with moving them from LED to LED.