Setting up my first HD AR, and lights are the next thing to look into.

I've seen some people mount them on the sides, some on the angles, and some on the top in front of the sight (like the one below).

Is this done for a reason? Or is it all due to ergonomics and being ambidextrous? I've seen this argument for placing pressure switches on the top (activated with thumb of either hand).

Wasn't sure if there was any other reason, perhaps the location of barrel shadow being in a better location?

Thanks

You pretty much got it. Ergonomics, economy of space and ambidexterity. Barrel shadow can be mitigated better by pushing the light out than picking which clock position to mount it.

If my sights would clear it, I'd put my light on the top to cast the shadow of the barrel/suppressor below my line of sight vs. to the left or to the right.

I'm trying out a top-mount configuration and am liking it so far.  No shadow, low profile, and lots of flexibility if you use a remote switch.  This is my latest Cali legal build - weird grip, pinned stock, thread protector; still playing with the switch position...