A Delta employee recently explained the standby policy in a metaphor-type of a way, which helped explain it a little better. It goes a little something like this...

You walk into Macy's and buy a green sweater. A few days later, you decide you don't want that sweater, but you want a different type of green sweater. So when you ask the employee if you can exchange your green sweater for the green sweater on the display rack, the employee doesn't let you because it's not the same sweater. You say, "what?!?! It's simply an exchange of a green sweater for another green sweater, I don't understand." The employee responds, "no, the two sweaters are two separate products".
That's it in a nutshell. Referring to Delta, each seat may seem like the same seat, but they are sold as separate products with separate price tags and restrictions attached to it.

Of course, as a gold and above, that analogy just flies out of the window...