Most judges simply don't have time to try every case that comes through the door. For a prosecutor, the judge's concerns about a clogged calendar are the prosecutor's concerns as well. When the judge is bogged down, the judge puts pressure on the prosecutors to move cases along quicker. Of course, prosecutors are also concerned about their own calendars. Crowded calendars mean that the prosecutor's staff is overworked.
Plea bargains tend to lighten everyone's caseload.
Because plea bargains are much quicker and require less work than trials, they are also easier on the prosecutor's budget. With today's cutbacks on already slim resources, prosecutors feel they will have additional time and resources for more important cases if they conclude a large number of less serious cases with plea bargains. Another benefit to the prosecution is an assured conviction. Plea bargains also give prosecutors flexibility. For instance, they can offer a deal to someone whom, though guilty, has given testimony about a co-defendant or helped resolve some other troubling case.