I needed to replace a leaky bathtub faucet.  I have to do this because the rubber seal will eventually fail.  I bought a new one and I needed a toll called a Faucet seating wrench.  They are available at hardware stores.   Mine was square and there are also Hexagon ones.   I used a caliper to measure out 8mm is what I needed. This piece is past the faucet and seats behind the stem and you cant reach it without a tool.   This was just a simple fix, but I am sure others could use it.