If you "need" to yell at your kids

That's a little quick to judge. I agree that parents shouldn't always be yelling at their and feel that doing so it necessary, but there are things that do warrant a stern raising of the voice. Such circumstances may be few and extreme, but I wouldn't think less of a parent for yelling in those situations.

I don't know a single parent who haven't yelled at their kids at one point.
The difference is if they acknowledge the effects, apologize and try to work on resorting to other options when the 'need' arises.

Or they are out in the back 40 and it's time to eat.

some parents spread layers of @#$@# one after another on their kids to cover up their own failings

but also the childs nature will always have an effect on how things turn out

Some kids are so thick headed you need to yell to get their attention.

It must be hard sometimes because kids are savages