Many states require bail agents to prove they are able to pay for forfeited bail bonds. A surety company is essentially an insurance company that will guarantee this.
In most cases, surety companies will require a certain percentage of every bond written. A percentage of this goes to the surety company and a portion of that goes into a build up fund (or BUF), which helps pay for forfeited bonds.
Many sureties also provide support and added value to help a bail agency run with ease, which is why it is important to find a surety company or general agent that is right for you.