Think of ships like submarines.
I'm in one sub and you are in another. You want to kill me but you need to get close. I want to find you before you get close.
There are two ways I can find you
1: I can listen for you (passive)
2: I can emit a "ping" and listen for the echo of the ping bouncing off you. (active)
Listening for you doesn't give my position away. I can listen all day and you won't see me listening.
"pinging" does, because you can see where the ping is coming from.
Listening for you doesn't give me a perfect picture of where you are (there's other noise, your noise echoes)
"pinging" does
In aurora, weapons cannot target unless a ship is "lit up" by active sensors.
most spacefleets don't just sit there "pinging" all the time. The enemy would see them too easily. usually, they just listen, and only start pinging when they KNOW something is close.
If you want to make a ship that can't be heard by ships that are just listening, you need thermal reduction
if you want to make a ship that can't be seen by ships that are actively pinging, you need cloak
if you want to make a ship that can never ever be seen, you need both.
It is never perfect. You'll ALWAYS encounter some situation in which you are seen (for example if you fly past them at 10,000 KM and fire your guns, they'll see you, even if they are very primitive., but if your tech is high enough, you'll almost never encounter those situation. If you're trying to make a bird of prey type ship that closes to within 10 million kilometers and fires a ton of photon torpedoes, you'll want both thermal reduction and cloaking.