It gives a microcontroller 16 extra push-pull I/O lines over a two-wire bus, with a power-on reset (POR) that forces all outputs low at start-up — no floating pins during boot.
The push-pull outputs can source 10 mA or sink 25 mA per pin — enough to drive a standard LED directly (add a series resistor) or pull a relay coil through a transistor. If you need more current, the 25 mA sink rating is per-pin; the package total is the ceiling, so don't parallel outputs to exceed it.
Interrupt output and POR — design-in gotchas
The interrupt output (/INT) goes low when any input pin changes state, which lets the MCU sleep until a button press or sensor edge wakes it — saves power in battery-backed panels. The POR feature means all I/O pins come up as inputs (high-impedance) after power-on, so no output glitch on a load that expects a known state. One gotcha: if you need the outputs to be high at reset, you'll need external pull-ups or a different expander that defaults high.
Package and footprint — 24-TVSOP vs 24-TSSOP
It's a surface-mount footprint that matches the standard 24-pin TSSOP land pattern, so if your board already has a 24-TSSOP footprint for another expander, the PCA9555DGVR drops in.