Diagonals are the highways of the bishops and queen. A bishop placed on an open long diagonal can rake across the whole board, exerting pressure on distant squares and pinning or skewering enemy pieces. Fianchettoed bishops on g2, b2, g7, or b7 are specifically developed to dominate a long diagonal.
Controlling a key diagonal pointing at the enemy king is one of the most reliable ways to build an attack. Because each diagonal sits on a single color, a side that loses control of one color complex often finds its opponent's bishop and queen operating unopposed along those squares.