I had a leash when I was a toddler.
I think they're great for when you're in the public and you don't want the kid to run about and getting in the way of others.
And it's for their own safety - I was with a relative when her daughter wandered out our sight at a crowded shopping mall. The panic that instantly rises up in you is horrible. Thankfully we found her daughter within a few minutes, but I wouldn't want to have to go through that again!