Minecraft wiki entry for Anvil, associated text: If an item is renamed, such as diamonds, it cannot stack with unnamed or differently-named items.
For unenchanted items, "unit repair" can easily cost more material than just crafting a new item or combining damaged items. The exception is armor, where players can use less material at the cost of experience levels.
If placed on top of exploding TNT blocks, the explosion does not affect the surrounding area. This is because the anvil falls into the space the TNT entity is occupying, and since the TNT's explosion power is not high enough to destroy the anvil, no blocks are destroyed.
In Java Edition, any item can be enchanted with the anvil in Creative mode.