I think I was wrong once!
I too don't understand why to check for bad ground or bad bulb. I would say it's a bad flasher relay, if lighting works, hazards flashers work, then ground and bulbs should be good.Gosh, now I'm confused.
If the bulb burns out, it flashes faster. If you install an LED, it flashes faster because there isn't enough resistance. So now we check bulbs for a flasher that comes on but doesn't flash?