Ugh. Why.
There may be evidence they can’t get into the trial via other means. I can’t think of why he would take the stand; but maybe given what has transpired he thinks he will come across as sympathetic. IANAL but usually this is risky. One slip up and he can doom himself.
In the defense opening argument, Rittenhouse’s legal team told the jury that they’d be hearing from the defendant. If they didn’t put him up there then the defense team might think the jury will count it against him.