Your anvil can be upgraded 2 times. The first anvil allows to to repair tools up to diamond. The second stage lets you repair diamand tools. The final upgrade enables you to fix your enchanted items. The anvils are costly to make, requiring a lot of iron, diamonds and eyes of ender but they are definitely worth the cost. It takes 8 diamonds all together to build the best anvil, but you have to consider how many diamonds you save when repairing your items.
Once you have placed the anvil, you are able to pick it back up like other blocks. This is good if you want to carry it round on a mining trip for example. Even if you aren’t on a mining trip, it’s still useful to have with you in case your weapon is almost destroyed and you need to fight your way through a path of mobs. This mod also works with a lot of custom weapons that can be damaged as it does not modify the base classes.