I've noticed that I often find questions people left on a forum that somehow go unanswered, and I have thought, "I sure wish someone had answered that guy cause I wanted to know too." Or the person will write back and say, "never mind, I found the answer", and end the thread without offering an explanation. Then of course a year later, there's this thread sitting on the forum that contributes nothing to the discussion. After all, I'm happy that you figured it out, but maybe I wanted to know too.
So, while I was waiting for someone to explain to me what the purpose of tags are in NexttGen galleries and albums, I ran across a start of an answer. I was trying to apply them to the normal situation of using tags from within the web site in a similar fashion to how tags are used for finding posts, and wasn't getting anywhere.
It turns out that they work the same, but specific to images. In their simplest form, you can use tags to search out individual images you have scattered around your various galleries. But when you drill down a little bit, their true abilities come to light.
Tags, it seems, can be used to generate truly dynamic kinds of galleries and albums. Here's a brief explanation of how they work here:
Let's say you have a gallery of Yellowstone, one of Rocky Mountain National Park, and one of Glacier National Park. And within those galleries, you have images of elk, and you have those images tagged as "elk" among other things. Now, if you wanted, you could create a gallery using shortcode and tags, looking specifically for elk, or a list of other critters, or just critters if that's in the tags too, and you'd have a gallery based on tags that span across all your other galleries.
Of course, the next step would be to create a searchable gallery where a user could search the images themselves, but I'm not quite at that stage yet. Later maybe. Actually, now that know it's possible, there's probably something out there that tells me how to do it..