Why?  I keep seeing ads for them and I just don't get it.

In my opinion, a fast-fading current fad and another answer to a question no one asked.

In a few years, like radically hooked trigger guards and other past fad options, people will be wondering what we were thinking.

Aperture, "peep", or "ghost ring" sights only really work when your eye is very close to the rear sight.

it seems like it would block your view of the target when you're holding it at arms length...

Take off the top half of the ghost ring and you might have a decent set of sights.

I think they fill a nich.
They are fast, but not accurate and are a good choice for someone who shoots thier rifle/shotgun with ghost ring sights a lot and wants the back-up handgun to have the same sight picture.

Ghost rings on pistols are teh suck.

I used a set from JP called double rings for competition for 3 yrs. They are faster for target sighting but not as precise and notch / post. For shooting fast to 20 yards at man sized targets they are quicker then most would think. Very few have actually tried them for any length of time to truly appreciate the benefit. as you become a better shooter you will outgrow them so to speak. I have a set on an old 45 that i let new shooters use and most will say it's easier to sight a target with rings than notch post when they are new.