Jail houses criminals. Now, jail is separate from prison, which holds criminals serving long-term sentences. But jails still do house criminals serving short-term sentences and those awaiting their trial. These guys may not be the most dangerous of the bunch, but they still do have some intimidation factor, even if simply being labeled a criminal.
For most families and friends who have a loved one in jail, that idea is enough for them to shudder. If there was anything they could do to prevent their loved one from being in custody as much as possible, they would do it, like getting a bail bond to bail them out of jail.
Bail bonds release defendants from jail, but only defendants who are still in trial. Bail bonds do not apply for anyone who has already been found guilty and is serving a sentence. This is the most a bail bond can do, and this is certainly good enough.