The reason to know how is simple. It saves money and a trip to the ER. Why spend a few hundred dollars and waste the hours when you can do it yourself in 15 minutes?
I don't think the people wanting to know have any delusions of sewing limbs back on or knitting up chainsaw wounds. This is only useful for medium depth clean edge cuts. Otherwise if you're not going to see a doc the best thing to do is leave it open, keep it clean with frequent bandage changes and take antibiotics if you have any. It'll heal this way, albeit with an ugly big scar. People did survive for thousands of years without doctors and they did it by doing shit themselves.
And I only stitch myself, family and friends. Never did while on the job as an EMT