A: The LHC will produce large amounts of experimental data. It will be necessary to compare the data with theoretical predictions. I am involved with building models of new physics and coming up with ways of testing such models using the LHC data.
A: Current particle physics models rely on the assumption of the existence of an undiscovered particle called the Higgs boson. One major goal of the LHC is to discover this particle. But, it would be quite surprising to see just a single Higgs boson. We expect to find many new particles. Theorists have come up with several suggestions such as extra dimensions. The LHC data finally will tell us which ideas are supported by evidence, and it will let us probe deeper into the underlying nature of the universe.
A: While the primary goal of the LHC is to enhance our knowledge of the physical world, the engineering efforts to build such a pioneering device will have useful spinoffs. For example, the improvement in magnet technology from building the LHC can be applied to other medical and consumer technologies.
A: For the last couple of decades, we have been just short of the experimental technology to finally settle the nature of dark matter and the Higgs boson (primarily because of the difficulty of building these colliders). The LHC will definitively answer these questions and will transform our understanding of particle physics.