The first character is the sheriff, Heck Tate. Mr. Tate is called to tell about the day he went to see Mayella, and he tells about what he say in regards specifically to her injuries. Having Mr. Tate goes first sets up the tone of the trial. Although much argument goes on, there is not a sense from the outset that the goal is to convict Tom Robinson. Mr. Tate's testimony is balanced and unbiased. He does not have a prejudice against Tom, and so the readers begin the trial feeling like this might turn out in favor of Tom.
If Mr. Ewell had been the first to testify, the tone would have been very different. An early bias against Tom would have been established, and Atticus would have been more on the defensive and not so much in control.
The trial represents both prejudice and change in the world of Maycomb - prejudice because an obviously innocent man gets convicted, and change because it takes the jury time to come to that conclusion. They don't automatically condemn the black man. Having Heck Tate go first opens the door to the idea and the hope of change.
Sheriff of Maycomb County Heck Tate is the first witness to testify in the trial of Tom Robinson, the African American man accused of raping a white woman--an extremely serious charge in the American South during the period in which To Kill a Mockingbird is set. The reader is first introduced to Heck Tate in Chapter 10, when he arrives at the Finch family's neighborhood to enlist Atticus's help in shooting a rabid dog. It becomes clear in this introduction to Sheriff Tate that he is a decent human being, but the full measure of his integrity as a law enforcement official and as a human is only revealed in later chapters, as when in Chapter 15, the sheriff visits Atticus at the latter's home to discuss the possibility of moving Tom Robinson to a safer location until the trial and. Heck Tate is shown to be a good person again when the lynch mob that Atticus confronts in front of the jail informs the lawyer that he, Atticus, should not hope for the sheriff's assistance in defending Tom Robinson from the mob, as the lawman was diverted from the scene by a fraudulent call. (The implication is that Heck Tate would indeed have come to Atticus's aid if he could have.)
With Heck Tate's character established, the reader can assume that his testimony before the jury in Tom's rape trial will be impartial and professional, and it is both. As Chapter 17 begins, Sheriff Tate has taken the witness stand. His testimony is straightforward and he sticks to the facts as he knows or understands them, which is inadequate given that the key accusation is made by the "victim's" father, Bob Ewell. In any event, it is the sheriff's testimony that allows for Atticus to establish that Tom, whose left arm, as described by Scout, "was fully twelve inches shorter than his right, and hung dead at his side. . .[and] ended in a small shriveled hand," is physically incapable of inflicting the wounds Mayella Ewell received. The direction of those wounds could not, Atticus points out, have been inflicted by a man without a functioning left arm.