The Demolish is the stock PDA weapon for the Mechanic. It allows the user to destroy any existing buildings that you've built. It is better to manually destroy a building rather than leave the area to haul it or wait for someone to destroy it. This is extremly important when doing rollouts as a teleporter saves progress as long as only one of the teleporters remain alive at the time. Overall it is nothing but a tool that helps out a small bit.