The first browser tools were a Firefox plugin called Firebug in 2005. It revolutionized debugging in the browser. At that time browsers did not provide a stack trace to errors. Error messaging was obscure and not helpful so you really had to guess your way through debugging.
Firebug also allowed live adjustments to CSS, which did not exist before.
Firebug also allowed live adjustments to CSS, which did not exist before.
That’s the back story.