Why do judges impose suspended jail sentences? What exactly does that mean? If a person doesn't have to serve time in jail, why is the sentence imposed at all?

A "suspended sentence" is one which is imposed on a defendant but not implemented or executed--meaning, if the person stays out of trouble, he does not have to serve the sentence. Judges often impose suspended sentences on first time offenders. It is an incentive for the defendant to stay out of trouble. And it preserves scarce jail space for more serious offenders from whom society needs to be protected. Let's say you are charged with shoplifting. You have never been in trouble before, and you decide to plead guilty. Or, you think you have a good defense, and go to trial, but a judge or jury finds you guilty. When it comes time to sentence you, the Judge may feel your offense was not so severe as to warrant jail time. But she may want you to have something hanging over your head so you will think twice before committing another crime. A suspended sentence serves this purpose. The judge imposes a sentence on you, for example, 90 days, but suspends execution of it, provided you do not get in trouble for a usually longer period of time, say a year. At the end of the year, if you haven't gotten in trouble, the sentence is lifted. But if within the year, you do re-offend, the judge can make you go to jail for the length of the suspended sentence, here, 90 days. Again, the sentence is imposed as a deterrent to keep an offender from committing another crime. The thought is that with a suspended sentence hanging over your head, you will think twice before doing something illegal.