Anyway, each side gets to tell the fact finder: this is what we think the evidence will show. Events occur chronologically, but evidence doesn't always come in chronologically. A witness will testify only once, usually, and they'll have to say all the relevant things they know, whether this comports with the timeline or not.
The opening statement gives the framework into which the various pieces of evidence will fit. It frames the picture that the evidence should paint.
If someone starts making an argument, you can object: argumentative! There's no rule that defines this objection, incidentally. There are many objections not reflected in the rules themselves. Some rules are still common-law, in other words.