"Is there anyone here who uses it?"
Yes
"What are some of the things that you can see on cards that aren't visible under normal light?"
Many post-WWII paper products (and, really, lots of other products) are made with optical brighteners. These chemicals make colors stand out more, but they also glow under a black light. Shine a black light on a white t-shirt and you'll see what I mean. If a card that purports to be from pre-WWII glows brightly under a black light, that's a strong sign that it's a fake. (Many things glow a little naturally, so you might also want to compare a suspect card with one you know to be genuine.) You might also be able to spot things that have been added to a card later (like re-coloring). Note, though, that not all newer materials have this stuff, so it's not a fool-proof test.
"Do you think that it's important to own one, especially for collectors who like to spend big money? "
I don't spend big money and I have one.
As for the specs, I don't know. Most of the little flashlight ones release a little visible light also. That's okay, but it makes it a bit harder to see the effect of the black light. Ideally you want it to be really dark.
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