I'm assuming you're referring to usage on Scizor, since you have it in the tags. Knock Off gets a 50% power boost if the target has an item, giving it 97.5 power the first time it's used on an opponent as opposed to Thief having 90 with Technician. And more importantly, it allows you to remove items on your opponent's team without sacrificing Scizor's own item slot (or restricting it to very specific items that get consumed quickly, which Scizor usually doesn't like to run if it doesn't need to). It's less useful for the damage alone and more used for utility.
