At the beginning of Chapter Seventeen, Sheriff Tate takes the witness stand and testifies in front of the court. When Atticus asks him to describe in detail the events of November 21, Tate gives his straightforward account of what happened when he arrived at Bob Ewell's home. Although Sheriff Tate initially says that Mayella's left side of her face was beaten badly, Atticus helps him recall that her right side was actually beaten. Tate also testifies that there were bruises around Mayella's throat. The reader gets the impression that Mayella was savagely beaten. At this point in the novel, the reader is not aware that Tom Robinson's left arm is not functional. However, Tate's testimony is presented first in order to give the reader a clear idea of what happened after Tom Robinson left the scene. Tate is a reputable witness and is telling the truth, unlike both of the Ewells. 

Mr. Tate's testimony is very important to the reader because it establishes a point that Atticus works on making for three chapters. But you see, the point was never something that Atticus bargained for from the beginning, Tate helped Atticus figure something out which helps demonstrate that Atticus is not the only moral person in the courtroom. Tate was helping. Mayella was beaten with someone who lead with their left. Depending on Tom Robinson's strong side, this creates doubt and suspicion if he is not a lefty:

Mr. Tate said, “Oh yes, that’d make it her right. It was her right eye, Mr. Finch. I remember now, she was bunged up on that side of her face…”

Mr. Tate blinked again, as if something had suddenly been made plain to him. Then he turned his head and looked around at Tom Robinson. As if by instinct, Tom Robinson raised his head. Something had been made plain to Atticus also, and it brought him to his feet.

We can also conclude that adding this evidence to the testimony of Bob Ewell will put him in position to be suspected as the one who beat Mayella.