The theory behind airline mergers is that they create economies of scale. Back office operations can be chopped in half. Airport gates, most of which are expensive, can be consolidated. Planes can be taken out of service, and some duplicate routes can be cut. Staff reductions save employee expenses. The Delta, United and probable American mergers have each been set in turn to create the largest airline in the world. The size of an American tie up with U.S. Airways will create the largest one of all.
The theory behind merger effects does take into account affects on product offerings but, in practice, these are difficult to measure. This difficulty becomes apparent in Luke Froeb's contribution "Post-merger Product Repositioning:."