I'll give you some reasons.
1. You might want to keep your OS, programs, and other files in tact. You might just want to wipe part of the disk.
2. You might want to wipe 5+ drives at once on the same machine, all while working on other tasks, on the same machine.
3. You might want to wipe your network storage devices that houses 100 drives, and I don't think it would be a good idea to restart a server to do so...
4. You might want to wipe all of the sensitive folders and all of the deleted data on everyone's C drive at once, even if there are 20,000 machines in the company. It would be ridiculous to reboot all 20,000 machines. Let's see you use your old technology for that. ;) In fact, let's see you repeat the process every month. Suddenly Cipher is a breeze in comparison... Set it up once and click a couple times for every time after that...
You might be able to do the first with a bootable disk, but you won't be able to do the second without using an entirely different machine.
The methods are comparable, but, at the end of the day, Cipher will allow you to use a machine while wiping extra drives attached to it. Not only that, but it goes over the disk 3 times, and you can run it as many times as you like for even more passes.