Sure, back when I still went to comic shops. That's how I got my Teen Titans #1.

Cei-U!
I summon the olden days!

They're usually too expensive for me, so not often. Once or twice over the years, when I've seen something I'd been looking for for awhile and it wasn't too crazy a price.

I rarely do this anymore, although I used to buy a lot of "wall" books back when I was trying to flesh out certain series that I cared about. Almost all of my Captain America books from 100 to 120 are from the wall, as are most of the true keys in my collection (I use "true" to denote books that have long been considered key, as opposed to books that have become key due to recent or upcoming movie or TV appearences).

These days, however, I'm more for finding mid-grade copies of the handful of books still on my want list instead of dumping stupid money on a single book.

Only once that I can remember. The owner of the LCS I was going to at the time procured a (I'm guessing, I'm no expert) VF or so copy of Silver Surfer #15. At the time I was buying the Fantasy Masterpiece reprints of Surfer, and then the four issues that were not reprinted, at that time; #15-18. That issue was the last one I needed. It was the most, I've still yet to pay for a comic book, at $40 back in '96. But hey, those were the good ole days when 90% of my income was disposable instead of like 9% now.

Nah, I'm not interested in buying books just to have a valuable one. If there's one I want to read, it's probably available in a collected volume, or even a reprint comic. But sometimes I do look over them to see if there's something up there that I happen to have already, or something new that I didn't realize was valuable yet.

It's probably to either showcase high demand items that they have or, more likely, are part of a new collection they obtained.  Just like Target or Best Buy showcase new items that they know people want (iPhones, clothing items, etc.), a comic book shop owner needs to promote his merchandise, particularly if he has invested money into buying a back issue collection.  For every week those books sit in his shop, he is out money, so he will want to move it as quickly as possible.  It is in his best interest to show people, some of whom may be in his store week in and week out while others may just be stopping in, that he has books that are hot or are of some value in an effort to drive sales.

I worked in a comic book store in college, as well as managed one for six months after I graduated, and each owner had very different ideas about how to do this.  The one in college kept the same things on the wall month after month, even when he was buying new collections, and so for regular customers, they just stopped looking at it, because they knew the books never changed, and he rarely sold anything off of there.  In the store I managed, we rotated the wall stock every month or so, taking a handful of higher-dollar books out of the back issue boxes and putting them up there or adding things from collections as they came in, which made the store seem like a place to be on a regular basis, as customers never knew what new things they might find, even if some of the "new" things had been in the store for months.

Nah, not yet. = I sold some of my priciest books to the comic shop (Aquaman # 1, Cap # 100) and I was mad they didn't go on the wall. Jerks.