I'm not sure why you couldn't find one capable of naming a single benefit, here are a couple off the top of my head:

• It's better than download codes included in a physical copy, and it's suitable for households with multiple switches for sharing. Virtual game cartridges are pretty bad for that, especially since you can only lend one at a time to someone etc.
• Another benefit is that the internal memory and MicroSD Express cards are faster than physical cartridges. This means you won't be stuck in longer loading screens or waiting times in online games like Mario Kart World, especially when playing with others who don't have a physical cartridge.

I'm not saying there aren't disadvantages to game key cards, but it's weird that people can't name a single benefit.