Division of labor. Aircraft with more complex weapons systems/radars require an additional guy to process it all and make decisions.
Strike aircraft (think tactical bombers) often require a second guy to assist with the weapons guidance systems.
Fighters with limited weapons systems and a greater degree of automation for the weapons they do have are often single seat aircraft. Better computers allow for modern fighter aircraft to be single seaters (think F-35, F-16) while older ones with weaker computers needed an additional weapons guy in the back seat (F-14, F-4)
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