All vehicle exterior lighting functions are highly specified as to their design, construction, and numerous aspects of performance including light distribution: how much light they send out in what directions—which is determined by the bulb together with the lens and/or reflector optics. It's far more detailed than just "Well, it's a blinking amber light facing front, so it's a front turn signal". We do not get to just declare any ol' light into whatever function we want (unless the goal is to get hit in traffic).
For more detailed info on why ""Yep, that lights up and looks good to me" isn't even close to good enough, see
here.
