They recently published this video demonstrating the proper use of a snatch strap. It's important to use them correctly, because a mistake can be painful or deadly.
Most vehicle recovery equipment (winches, Hi-Lift jacks, etc.) are inherently dangerous, but anything that stores kinetic energy such as a snatch strap must be used with caution. It's always good form to keep bystanders out of range of potential flying objects.
A snatch strap should be considered a mandatory piece of emergency equipment (along with a clevis or two) that you keep with your off-road vehicle. It's not a replacement for a winch or a Hi-Lift jack, but it's lightweight, compact, and inexpensive so there's no excuse for not carrying one with you. Sometimes it's the only tool in your recovery bag that will work!
Here's a clear demonstration of the wrong way to use a snatch strap: