Daemons are never alone, but always connect to an existing hackable item, like a safe or a security camera. They only trigger once the firewall of the said item is disabled completely.
Here's a usual example:
Here we can see a Corporate Safe, guarded by 1 level of firewall and one hidden daemon. To open the safe, you need to crack open the firewall, but this will also trigger the daemon (which is revealed when triggered).
So, why would you hack a daemon? Maybe you want whatever is in the safe. Maybe you want to disable that pesky security camera. Maybe the daemon is on your mission objective and you have no choice but either to hack the daemon or abandon the mission.
There are ways around this. Sometimes you can knock the daemons out. Sometimes you can disable the device some other way. But most of the time, you'll either have to bite the bullet with the daemon (usually without knowing what it is beforehand) or just leave the item be.