One of the cornerstones of our justice system is that anyone accused of a crime is innocent until proven guilty. Another protection offered is to allow accused parties to resume their normal life until such time as a determination of guilt or innocence can be made.
In many minor instances, this can be something as simple as signing a promise to appear for your court date. In other, more serious, cases, some surety is required. This comes in the form of bail, wherein the person puts up a certain sum of money as a guarantee of their appearance.
Of course, most people don’t carry a large lump of bail money around in their pockets, which means that affordable bail bonds are the way in which they can both guarantee their appearance and not have to come up with substantial sums of money.