In general, any "waiver" is simply an agreement wherein someone gives up their right to do something. The right to sue over personal injury or some form of personal harm sustained during a yoga class is waived in most cases with a liability waiver. The risks are clearly explained in the waiver to each student before they participate in the class.
Like wearing a helmet when we go out for a bike ride, we don't anticipate crashing and hitting our head, but the helmet is there just in case it happens - and helmets have saved many lives! When the potential of personal injury exists, attorneys generally agree that a well-written waiver can be an important part of creating a sufficient barrier to provide you protection and peace of mind. Will it exempt you from ever getting sued? No, but most yoga studio owners find that having a liability waiver specific to their yoga practice provides peace of mind and even a higher level of professionalism when done right.
Nearly every attorney will strongly recommend a custom-written waiver for your yoga business, pilates studio, or training program. But why wouldn't they? They get to bill you for creating that custom document, right? That said, one of the challenges frequently seen with templates is that the waiver template doesn't properly address the specific inherent risks in your business and the template generally won't address the specific location of your business and specific laws surrounding that location. It may be worth investing a little to get your document right up front.