Yes, It is only the first step .To be clear however, a signed waiver in no way prevents someone from suing you, and if they can prove negligence, they will win. If you win, likely you will still need to pay your own legal fees. If you acted maliciously or criminally and injured someone, you can be criminally prosecuted as well. A waiver certainly is not a free license to act like an a-hole with impunity.
A liability waiver is a useful tool because it shows that the participant was aware of the chance of injury connected to the training. So if they sue you, the court will take that into consideration and might be more inclined to side with you, if it seems like you are being sued for a legitimate accident. It becomes one of the tools you can use in your legal defense.
This is why all instructors should carry liability insurance.
Legal fees are out of pocket, unless one can bring counter suit.
And yes Liability Insurance is required and used by the site / company .