Lets say that you have all of your presentations on your hard drive of your laptop and you have traveled to the other coast to give several presentations. While you are giving one of your presentations your hard drive crashes. You try to reboot but "failure to find boot able media".
If you have a bootable thumb drive with the OS, your presentation program, and all of your presentations, you just set the USB drive as first boot device, plug in your thumb drive. It boots and you continue with your presentation.
Catastrophe avoided, and your still have your job.
I think if someone was giving a presentation, it would be quicker and easier to borrow someone else's laptop. Reloading the OS can take an hour or two. It would make more sense to keep your data on the flash drive.