system
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human
ELI5: why do some fighter aircraft have a Weapons System Officer while others don't(?) (more in comments)

This is inspired by seeing Top Gun: Maverick, where one fighter has a second person in the cockpit as a weapons system officer, while the other doesn't. Does the other aircraft only have a one-seat cockpit?
gpt
There is advantages and disadvantages of having a crew of one or two in a fighter. The goal here is to divide the work load of combat with the plane. The more work load there is the more having a WSO is important. But having a WSO also come with drawbacks since the plane need to make room for him. This can mean less fuel, more weight, etc. It also cost you more in training, you double the number of people you need for combat, etc. So usually you want only one pilot unless it's necessary. 

In general, a fighter designed for air combat won't need a WSO. Most of the weapons are highly automated so the world load is usually manageable for just one pilot.

That said there is some exceptions. For example, the F-14 Tomcat was mainly an interceptor/air superiority fighter, but it had two seats. The reason was that the Tomcat main weapon was the AIM-54 Phoenix which was a long range anti-air missile that used a semi-active radar guidance. The way it work was that the Phoenix missile needed the radar of the F-14 to guide itself to the target at Beyond-visual range. This mean that the WSO had to be concentrated on the weapon during the long flight while the pilot took care of the fighter. Needing the radar of the fighter to be on is like a big blinking arrow that show to everybody where the fighter is, so that's not ideal in combat. Modern radar guided missile are usually active now, meaning they have their own radar. For that reason, the work load is lower on modern fighter and they are usually single seat.

A fighter designed more toward ground attack will tend to have a WSO. The reason is because targeting something on the ground is far harder to do than targeting something in the air. There is a lot more stuff to hide on the ground, there is a lot more possible target and it's harder to identify if it's a friend or a foe. For those reason, some weapons add enough work load that having a WSO make a big difference.

Not all ground attack weapon need the same amount of work and the latest technology simplified a lot of task too. For example the F/A-18E is the single seat variant that is mostly used for air combat, while the F/A-18F is the two seat variant that is better equipped for ground attack. But the F-35 only have one seat variants because it have a lot of technology to help the pilot. The sensors, avionics, helmet-mounted display, and the targeting equipment like the EOTS.

TLDR : It always depend on what weapons the aircraft will mainly carry. Some weapons give too much work load for a pilot alone. Rule of Thumb is Air combat need only a pilot, while ground attack need a WSO. But that's just a generic rule, it depend on the specific design of the aircraft and the technology.