Sorry in advance, this ended up longer than I expected.
I can't say much about why other people are interested in them, but here's how I look at it.
The set is popular. Probably because it's large enough to have subsets that can be put together without spending a pile of money. Maybe a small pile if the subset has more HOF players or a tougher card. Even the set less the handful of cards that aren't all that affordable is possible on a fairly tight budget.
But the things that make that possible also make it frankly a bit dull. As the Ruttles said "all you need is cash" (and a bit of time).
So if you collect T206, and maybe get to a point where maybe you're between cards that are decent deals
and visually appealing, you start looking for something else. At first it's backs. But well, as interesting as they are, they're backs. And either you get the easy ones pretty quickly, or it just gets old. I started wanting one of each brand. And got as far as needing Drum and Uzit and ignoring the Ty Cobb back, until Drum and Uzit became impossible for me too. Then I wanted to get closer on all the factory and series differences. And found that I only needed about 6-7 of them. And that about half of those were out of the question as well. (I still have no idea how I missed SC150 F649
but I had. )
But.........I do collect printing errors on pretty much any card. Mostly modern since they're cheap. One dealer used to give me a few now and then. They're worth a bit if they're really bad, but not a whole lot especially if it's not a superstar.
And then I realized that T206 has this huge puzzle. Not just what cards came with what exact back, but how many were on a sheet. Since I also collect stamps, it seemed like a familiar puzzle. Stamps have a thing called "plating" where someone buys a boatload of stamps and tries to figure out which plate each one is from . Not done much on new stuff, they're very consistent. But on the 1850's stamps it's really popular. (Most have been done by now) It's also done for a few other groups.
T206 is a bigger puzzle, basically almost nothing is known. With stamps the size of the sheet is known, and how many different plates were used. And there are usually blocks or pairs. So you can figure out that the one with the slightly different corner is next to the one with an extra dot, and THAT one is the first one in the second row...........Yeah, it's a bit nuts. Just like collecting pictures of dead guys on cardboard is a bit nuts.
So, it's a huge challenging puzzle.
Other sets are the same, but for instance, I can find a picture of a sheet of T-212s. And the E cards and other T cards are a bit more challenging because they're usually more expensive. (I do save scans of some if there's an interesting flaw, or something like that. )
Most of the "finds" I don't think of as having any extra value. If a lot of people decide they want to pay a bit more--or a lot more, I'll be happy if I have one. If not it doesn't bother me. I gave up on the concept of "completing" the set years ago. But a new twist or tiny difference that can be shown to be consistent is a clue to the solution to the puzzle. And that's interesting.
And as a bonus, it's a challenge that doesn't require a whole lot of money. Or if there are good enough scans, any money.
There's also the little bit of thrill of discovery and the hunt itself.
To me it's a great feeling to hold something and realize that there's something special about it that's gone unnoticed for a very long time. And that the handful of people who care will be just as excited to learn that little thing.
I examine new cards the same way, I collect stuff that pretty much gets ignored by very nearly everyone. like different holograms on UD cards, and different die cutting on 1988 score.(And different screening on 1988 score that I just found out about. )
I'd probably be looking for them if the cards were still around 1.50 each like they were when I started collecting them.
If someone doesn't get it or thinks it's frightfully boring (and it IS, most of the time) I think that's just fine. We all collect differently for different reasons, and that's pretty cool.
Steve B