Every time I see a video of a supercars, the headlights can be seen flashing. I understand that this is most probably due to the fact that the LEDS are not always and the camera can detect this time but why are they not always kept on?
They flash because that's how their brightness is altered. Rather than use a large resistor to vary the current through the LEDs, they are flashed on and off faster than your eye can detect. When they are on more than they are off, they are bright. When they are off more than they are on, they are dimmer. That's a very general description, I don't know what the duty cycle (numerical description of how on they are vs how off) is for automotive LED headlights.
But the answer to your question is the reason they are not "just on all the time" is that it's harder, and more expensive to adjust their brightness level that way. It also generates more heat that way. The LED gets to cool while it's off.
You see this in the image because the camera is faster than your eye and is also taking the video frame by frame. So when you slow it down, you're catching things your eye isn't fast enough to catch.
I hope that helps!