You can add me to the category of "I would have never used one" before I had kids to "Yep, some kids need them!" after kids. I also thought they were degrading. I haven't used one before, but I've certainly stopped questioning why a parent would ever opt to!!!
But like so VERY many things everyone thinks they know about, you can't judge until you've walked in someone else's shoes or been in the same situation. And even then - every parent has different children. I know of children who listen very well, aren't prone to running off, and are content to stick close by, or sit quietly and play. I also know children who are wild for adventure, and very hard to keep track of, especially when you're looking after more than one child.
Yes, it's important to teach them boundaries, but the fact is simple it is just plain easier with some than others. And until you feel comfortable, and until they know the rules, you use a harness if you have to. People will judge and stare - but they do anyhow, with every aspect of life it seems. Shake that off. There's no feeling more sick than a kid who runs around the side of the car thinking they're playing a game with you in a busy parking lot.
Young children aren't always easy to teach and guide, and sometimes you need some tools to help you do it. Soldier on

Do what you need to do to keep your kiddos safe.