A discussion of the most common reasons why a defendant might want to enter into a plea bargain.

As criminal courts become more crowded, prosecutors and judges feel increased pressure to move cases quickly through the system. Trials can take days, weeks or sometimes months, while guilty pleas can often be arranged in minutes. Also, the outcome of any given trial is usually unpredictable -- but a plea bargain provides both prosecution and defense with some control over the result.
For most defendants, the principal benefit of plea bargaining is receiving a lighter sentence for a less-severe charge than might result from taking the case to trial and losing. In addition, defendants who are represented by private counsel can save a bundle on attorney fees by accepting a plea bargain. It almost always takes more time and effort to bring a case to trial than to negotiate and handle a plea bargain.
There are other benefits as well:
Getting out of jail. Defendants who are held in custody -- who either do not have the right to bail or cannot afford bail, or who do not qualify for release on their own recognizance -- may get out of jail immediately following the judge's acceptance of a plea. Depending on the offense, the defendant may get out altogether, on probation, with or without some community service obligations. Or, the defendant may have to serve more time but will still get out much sooner than if he or she insisted on going to trial.
Resolving the matter quickly. This has the intangible benefit, touched on above, of providing resolution to the stress of being charged with a crime. Going to trial usually requires a much longer wait -- and causes much more stress -- than taking a plea bargain.
Having fewer or less-serious offenses on one's record. Pleading guilty or no contest in exchange for a reduction in the number of charges or the seriousness of the offenses looks a lot better on a defendant's record than the convictions that might result following trial. This can be particularly important if the defendant is ever convicted in the future. For example, a second conviction for driving under the influence (DUI) may carry mandatory jail time, whereas if the first DUI offense had been bargained down to reckless driving, there may be no jail time for the "second" DUI.
Even for people who are never rearrested, getting a charge reduced from a felony to a misdemeanor, or from a felony that constitutes a strike under a "three strikes" law to one that doesn't, can prove to be a critical benefit. Some professional licenses must be forfeited upon conviction of a felony. Future employers may not want to hire someone previously convicted of a felony. Felony convictions may be used in certain court proceedings (even civil cases) to discredit people who testify as witnesses. Felons can't own or possess firearms. And, in many jurisdictions, felons can't vote.