When I worked for the Undergrad Student Gov't CrimeWatch Escort Service at OSU way back when (it merged--and then was replaced by--the Student Safety Service administered by OSU-PD), we had van, bike, and walking patrols. Although most folks related us to the "safety in numbers" concept of escorting students across campus, we also checked buildings to find unsecure doors and windows, and other safety/security hazards.
In short, for several years, I was paid (no, not much!) to pretend I was a burglar on an entire campus a couple times per week!
Now why would I want to "impersonate" a type of criminal? Because I learned the easy, nondestructive ways burglars can gain access, such as merely going around checking doors and windows for one that was unlocked or was able to be "pushed or pulled" open against a defective/inadequate locking device, climbing, and/or finding other weird, creative paths to entry. I also learned how other people in the area often did not pay any attention to us (our uniforms weren't THAT recognizable).
It was a pretty good education, and included a lot of other "lessons" as well. Okay, I gotta tell this story:
I had already been counseled against saying "hi" to people (we worked at night, and my supervisors said I was undoubtedly "scaring people") and also about looking into dumpsters and trash cans, which I was told was unbecoming to our "image" (hey, you FIND stuff in there!), so my partner Chris and I were quite aware we were under scrutiny. However, we came across a window at a lab with a hose dripping some liquid outside, and the window was not closed too tightly. We decided we had to "fix" this unsecure situation (okay, mainly at MY urging).
We got the window open enough to climb in and found ourselves in a "restricted area." Uh-oh, this couldn't be good for our continued employment if some professor got mad or something...
As we were leaving, I couldn't resist taking a peek in the open room across the hall, and found a lab guy sleeping on a cot with a phone directory as a pillow. Hmmm. I couldn't resist. I took a "You Could Have Been Ripped Off" slip out of my pocket, wrote how we got in the room on the back, and left it lying next to his head. I didn't try to keep especially quiet or anything, as I didn't care if he woke up. He didn't.
(If anyone ever wonders about my disdain toward those who just don't seem to take security seriously, it's because I've seen THE WORST.)
I would say practicing, or talking about, how to sneak up on someone is just another way to learn how to guard against those who might sneak up on you. Same as learning about "knife fighting" even if you have no intention of ever using a knife...
But there are a few practical uses as well, even though they may be rare. It is not out of the question that someday you may wish to evade/hide from/escape from a criminal or criminals. You may find yourself in the position where you wish to observe something, too, and knowing how to get to cover or in a better viewing area would be useful. If you are being pursued by a superior force and you doubt you can outrun them, yet have a decent headstart, knowing how to "become one with your surroundings" might be a winning survival strategy.
But mainly, what bug said: it's fun!
Karl