$('#some-form').find('input').addClass('error')
Obviously you can do this in vanilla JS, but it's more cumbersome so you'll probably end up rewriting your own less complete version of jquery. There is some value to using a well worn library with public documentation that other developers can find/extend instead of a home grown solution.
Anyways, cool to see another alum here! I hope the department keeps it around for a while longer :)