The point where they really become useful is when you get to higher tier stuff that would take several different types of material to fix through the UI, as opposed to just the one type needed to make the repair kit. When in doubt most repair kits are at or around the same materials level as the items they fix, so if a kit takes iron it will probably work on any item made from iron as well. Meaning you could spend, say, ten steel making a kit to repair an item that would only cost five steel to repair the other way, but if you wait until it's almost broken then the normal way might cost fifteen steel, so at that point the repair kit is now worth using. Plus the materials cost of repair kits can actually end up higher than the cost of just repairing the item through the UI unless the item is near death.
So if you make a basic repair kit it won't work on an expert level item. The repair kits only work on items that are in the same catagory.
Basically each piece of equipment - armor, weapon, tool - has a catagory of power to it, from basic to legendary. The repair kit system is not exactly intuitive or user friendly, but it does work.
