You probably already know this but I'll pretend I'm saying something interesting anyway (-;
Random number generators aren't. What they do is generate a seemingly random sequence of numbers based upon a deterministic but difficult to predict algorithm. It's NOT like shuffling a deck of cards, it's more like having an extremely huge deck of cards (millions or billions of cards) preshuffled for you. Well, even if the deck is preshuffled you will be able to predict, at least the beginning of, the sequence eventually. So what we do is jump into this preshuffled deck at seemingly random places, i.e. difficult to predict. This furthers the illusion of randomness without doing any real shuffling, we call this jumping in point the seed.
People generally use the system timer because it's available & difficult to predict but you could use anything else availble to the computer that seems to jump unpredicatably like counting page swaps, or the rainfall in Idaho. Strictly speaking, time only increases in the computer but it will seem random enuf because: 1) How much the timer increases will be hard to predict. AND more imprtantly 2) The preshuffling at a higher points in the sequence should (if it's a good generator) bring you back to lower numbers (loop back to) relatively quickly. Therefore, in a good enuf sort of way, it will look like you're jumping into the sequence anywhere high or low.
Now the punchline... when debugging it's vitally imporant to be able to replicate errors. So with random number generators we either turn them off & generate a know sequence or jump into the preshuffled sequence at known points. And in the later case because the sequence is fully deterministic (known in advance from initial inputs) we can be sure that we hit the same problem numbers at the same place in the program. Thus allowing us to get to the problems in the program again & again. Also, if the same random# generator is used on different computers you can use the same seed & get to the same point in the preshuffle & see if or how the proggie breaks on different computers.