That is an odd power.
It does have a few uses. First, it's a good ambush ability. Where you can Hide one round and go invisible the next.
It's also good if pretty badly hurt, allowing you to take cover and gain attack avoidance.
And sometimes you just need that advantage on an attack roll.
It's also a really fun social ability, so you can just hide outside of combat. Enter the empty throne room or feat hall, sit in a shady corner, and then just wait.
Generally, in my experience, when you gain a power like that you will FIND uses for it. You will set up situations to take advantage of a power to justify having it. Opportunities will present themselves.
I was wondering how being invisible without being 'hidden' (because you didn't use an action to Hide) interacts with Perception/Stealth.
Being invisible without being hidden isn't that hard to explain.
Think of it at the table. The kobold wyrmcaster moves 30 feet and casts
invisibility. Where does the party fighter attack? The space the kobold was, because it hasn't moved. But making contact is hard because the kobold is still invisible.
On its next turn, the kobold moves. Where is it? Well, if it didn't stealth everyone heart it scuttle across the floor, saw bump into a chair, and watched the dusty settle. So they know what square it's in. Because it's not trying to be quiet.
Same thing with the warlock. It moves in shadow, becomes invisible, and you can still hear her breathing, shifting in place, etc. You haven't heard any movement or seen signs of the warlock's passage. There might be some blood dropping to the floor still.
So not hidden. No check is required to locate.