Think of a group of sentries: one hears something and points, another looks at it through binoculars, and together they know more than each on their own. That is how sensors cue each other.
In practiceA counter-drone system around a base works like this: an RF detector hears a drone's remote control, cues a radar for the exact position, a camera confirms that it is a drone, and only then does a jammer or weapon follow. The radar can stay silent until it is needed.
Fusion is more than placing sensors side by side. The value lies in cueing (one sensor points the next one in the right direction) and correlation (linking measurements into a single track). A passive RF hit cues the radar to a sector; the radar delivers an accurate track; EO/IR confirms and identifies. The result is a more reliable picture than any sensor on its own — and you keep your radar silent for longer (Doctrine & EMSO, EMCON) because the passive sensors do the first part of the work.