Eight more I/O pins over two wires
It runs at up to 400 kHz on the bus, so it keeps up with standard I2C and SMBus controllers without slowing the system clock.
The push-pull output type means you do not need external pull-up resistors on the output pins, though the I2C lines themselves still need pull-ups to the supply rail. The interrupt output (INT) lets the host MCU sleep until a pin changes state, rather than polling the I2C bus — a real power saving in battery-operated gear. The power-on reset (POR) forces all I/O pins to inputs at power-up, so outputs stay off until the firmware configures them; that matters if the expander drives a latch or an enable line that must not glitch.
The PCA9554ADBR comes in a 16-pin SSOP package with a 5.30 mm body width and 0.209 inch pitch. The 16-SSOP footprint is common enough that most PCB layout tools have it in the library. No thermal pad to worry about, no exposed paddle; just decouple the supply pins with a 0.1 µF ceramic near the package and you are set.
That makes it safe to qualify for new designs and long production runs.