In Harper Lee's To Kill a Mockingbird, what is important about the testimony of the first witness at Tom Robinson's trial, Sheriff Heck Tate?

In Chapter 17 of Harper Lee's To Kill a Mockingbird, Sheriff Heck Tate's testimony during cross-examination is very revealing and helps Atticus discredit both Ewells as reliable witnesses, especially Bob Ewell.

During cross-examination, one thing Atticus gets Sheriff Tate to reveal is that a doctor was not summoned to examine Mayella the night of the alleged crime. When Atticus asks Sheriff Tate, "But you didn't call a doctor? While you were [at the Ewells' home] did anyone send for one, fetch one, carry her to one?," Sheriff Tate answers in the negative to all of the above. This is extremely important because, in the US court of law, a defendant cannot be tried for a crime if no concrete evidence exists to prove the crime actually took place. We call this principle corpus delicti, which translates from the Latin to mean "body of the crime." For example, no one can be tried for murder if a body is not found to prove a murder actually took place, and no one can be tried for theft unless it can be proven something was actually stolen. Likewise, no one can be tried for rape unless it can be proven that a rape actually took place, and a doctor's examination serves as crucial evidence. Therefore, through cross-examination Sheriff Tate's testimony reveals that Tom Robinson had been imprisoned and is being tried illegally.

The second piece of crucial evidence Atticus gets Sheriff Tate to reveal upon cross-examination is that Mayella Ewell had been bruised in her right eye. Sheriff Tate at first can't remember which of her eyes was bruised, but after deliberation, he finally says, "It was her right eye, Mr. Finch. I remember now, she was bunged up on that side of her face ..." (Ch. 17). Even Sheriff Tate realizes the significance of his statement: only a left-handed person would have been able to hit Mayella in her right eye while she was facing her attacker; Tom Robinson has been crippled in both his left arm and hand since he was a boy.

Hence, as we can see, Sheriff Tate's testimony reveals crucial evidence proving Robinson's innocence, despite the jury's verdict.