People arrested and charged with crimes are usually given the opportunity to post bail so they can be released from jail while waiting for trial. Without the availability of bail bonds, those arrested would have to remain in jail while awaiting trial, making many jail populations in the U.S. totally unmanageable.
The availability and the amount of bail is determined by a judge. The more serious the crime, the higher the bail amount and more stringent the conditions. If a person poses a danger to the public or any individual or is a flight risk, the judge can deny bail and make that individual remain in jail. Judges have much discretion when it comes to bail and at least in theory take risks into consideration before granting bail.
Jail overcrowding is an issue throughout the US. The bail bonds system offers a means of having some control over the jail population. When a jail reaches capacity, the authorities must take some type of action to alleviate the situation. Often they release prisoners, who have been sentenced, early if they meet certain criteria, i.e., non-violent, misdemeanor, served a certain percentage of sentence, etc.
Without the availability of bail overcrowding would be such a problem that many prisoners would be released because there would be no choice. There is only so much jail space available and it is very expensive to house prisoners. It makes a lot of sense to let people out of jail while awaiting trial, especially since many people are found innocent or not given jail time as their sentence. Bail bonds also allow people to continue with their jobs and support for their families and to earn the money needed to pay fines.