Do I need 5V or 12V to power a LED light strip?
LEDs don't really have a forward voltage requirement. They have a forward current limit, and that in turn affects the voltage. Current limiting is most commonly done with a resistor, the value is chosen with simple math using the forward voltage drop and desired/maximum current. You can use almost any voltage you want until you reach the sparky levels.

However, the above only applies to raw LEDs. if you are using a manufactured product you must use whatever power the product calls for. If it wants 5V, you buy a 5V supply.

5V is standard USB bus power, so if the product is intended for use anywhere near a computer they will probably build it for 5. 12V is standard in vehicles. Both voltages are popular away from those markets, with the choice often as boring as it matches other things that company makes.