Can you please justify the use of this method? To me it seems much more complicated than direct geometry modeling also it is much harder to control and get acceptable results with. Same goes for curves method. I do not understand why people find it simple as curves are to hard to control and make look right. Also the resulting mesh density after conversion is excessively high. Also the speed of tweaking  is much slower than with mesh editing, while I spend a week modelling a hair with polygons, with curves I can't complete in month because of complexity explosion, so I have to delete everything and start from scratch many times.
Sure, I'll do my best to justify.
In short, if the goal is realistic hair for video games / real time application then hair cards is the most effective approach. Even though it's not an easy or fast workflow, it produces a far more realistic result than modeling a solid mass of hair.
I hope this explanation helps!
Hi, I'm not an expert like Kent, but I could add something else to the answer.
Geometry modeling is not realistic and controlling your model in complex situations is much harder.
This way it’s easier to work, especially if you are creating AAA modeling because curves are easier to deform and at the end you will have just a few curves.
Making hair with particles in Blender is a mess and controlling the flow it’s very difficult, and creating difficult patterns like those showed on the links are a horror show.
Also, if you are sculpting and do not want to retopology your model or your model is very dense in poly counts, having your hair as an alpha gives you more control, because the particle systems is already baked in the alpha images, so your computer don't slow down.
This technique works very well and to me was a game changer, just think it in this way, you could create organic stuff using box modeling or even hard surface modeling, but it won't be give you a better result than sculpting… so that means that sculpting it's easier than the other technique? Hell no! But it’s more powerful and has more tools and possibilities to create organic stuffs
Regards.