It's just the more humane way of doing it, since subjecting your fish to high levels of ammonia during the cycling will harm the fish. When the fish get stressesd out, it increases the chances of the fish sprouting out diseases such as
ICH.
Now I'm not too sure, but a lot of fish might die after the completion of the cycling period. In other words, you could save money. The biggest pian though, is finding the right kind of ammonia


It might also be quicker to cycle, since you have full control of the ammonia levels.
Basically, it boils down:
1) Putting in pure ammonia into your fish tank. Just the right amount so you have roughly 2.0 to 5.0
PPM after testing with a kit.
2) Then you wait (probably several days), for the ammonia levels in your fish tank to reach at least 0.25 ppm, and then you add more ammonia to bring it back up to 2 to 5 ppm.
3) You keep repeating this cycle. On another forum I frequent, they mention that you are well cycled if 2.0 ppm of ammonia reaches 0 in less than 12 hours.
I could be missing a whole bunch of other details, such as measuring nitrites. So don't take my quick guide as the "everything you'll need to know"