Good question! There's a lot of debate around this issue. Most views fall along a sort of spectrum:
- It's all in your head. When you talk to a spirit, you're really just talking to a piece of your subconscious self.
- Spirits are egregores; that is to say, they begin inside of people's heads, but attain a sort of independent existence based on human beliefs about them.
- Spirits are real and independent of you, exactly the same way that monkeys and rocks and coca cola are real. They do not rely on humans for their existence. They're still there when you close your eyes.
What do I think? Hard to say. For most of my time as a magician, I would have said that spirits were egregores. These days, I'm coming around to a more complicated position. You see, I do sort of think that spirits have their genesis in belief/thought/Mind, but I'm starting to think that trees and monkeys and coca cola do too. The thing of it is, those things are all figments of imagination too. God breathes creation into being moment by moment, that's why he's called Ehyeh Asher Ehyeh. That's a thing that humans can do to. Only we (and God) can create ex nihlo, bring forth something where once there was nothing, think things into being (make stuff up). Here's what I can say from experience; intent is very important, but it's not everything. If someone calls Lilith to help them get pregnant, it will not work. If someone calls Lilith to empower herself as a feminist, it may or may not work. If you call Lilith to curse an enemy to miscarry (you monster! seriously! don't do this), it may or may not work. But, if you call Lilith in front of an abortion clinic to curse pro-life picketers, it takes about 15 minutes for them to get arrested (for assaulting you; be ready to get hit). (I picked Lilith as an example because beliefs about her have moved so rapidly and vary so widely between communities that she makes a good example of problems in the "egregore" model.)
Now, on to your actual question, about names. I'm a big believer in the magical efficacy and power of names. Partly because I'm Jewish, and we're big on that. Mostly, though, it's because the book that most shaped my ideas of magic when I was first learning was fiction in which that was a main theme. (Don't you judge me! I was 13.) It's also just an aesthetic preference of mine; I like names and words and symbols and that sort of thing. I'm not much a of a drawer, and I'm the worst musician ever, but I'm actually an ok writer, so it's to my benefit to believe very powerfully in the power of names.
Here's my take on it. Different gods are different, but they're not as discreet as people are. (Actually, people aren't really as discreet as they seem to be either, but that's a different lesson.) Treating Hermes and Thoth and Mercury as interchangeable isn't so far from the truth. They're all closely related, and they've been syncretized for millennia. When you start to throw Ganesh and Anansi and Legba into the mix, then you're not really dealing with a god at all anymore, you're really just dealing with an idea, and a vague and confusing one at best. (And these guys are far more similar than many other gods who get lumped together. Pretty much the only thing Aphrodite, Ishtar, and Freya have in common is their lack of a penis, but lots of people try to equate them.)
Some people will try to sell you a line like "All gods are one god, all goddesses are one goddess." I think that's total bullshit for a couple of reasons. It's certainly true that, at the end (beginning) of things, there's only one God, but all gods are God only in the same sense that all demons are God, and all people are God, and all trees are God, and all electrons are God. Honestly, though, that truth isn't terribly useful (or maybe I'm just not enlightened enough to use it?).