In today's challenging legal climate, having waivers are an absolute must when teaching yoga. It helps limits liability should someone attempt to sue you, your company, or your yoga instructors. As a best practice, we get waivers from everyone, including minors which the parents sign on their behalf....and even your friends and family! There is a great post on all of the legalities of waivers from the Yoga Alliance that I recommend reading here.
In addition to help keeping us safe, we also use the release of liability to find out more about our customers. On our waivers (either online or on paper), our clients are encouraged to mark off their interests. This helps us create Smart Lists later to send emails that may be of interest to them versus our other clients. See our full post on segmenting clients.