Nikon’s 10-pin accessory terminal is a more capable remote/accessory port than a simple IR remote receiver. According to the answers, it can carry signals to half-press the shutter (activate metering/focus), fire the shutter, illuminate meters, provide power, and on some bodies support TTL-level serial communication used for accessories such as GPS.

In practice, that means 10-pin accessories can offer more control than basic remotes, including wired releases and some advanced wireless/PC-controlled options. Simple IR remotes mainly trigger the shutter; camera settings and drive modes still have to be set on the body.

Why some bodies have it and others don’t is mostly a design tradeoff: extra cost, space, and complexity are more justified on higher-end bodies where advanced remote control and accessories are more likely to be used.

The Nikon D90 does support an accessory terminal for remote releases, but it is not the classic Nikon 10-pin connector. It uses a different connector that provides similar functions.