This practical rule only applies when a trial attorney is cross-examining a witness at trial. The attorney is not trying to learn anything during this cross-examination that he or she doesn't already know. The attorney is building their case or taking down the opposition's case by having a witness admit or state a fact so the jury, or judge in a "bench trial", can hear the testimony as evidence.
The reason that the attorney must NEVER ask a question which he/she doesn't know the answer to is that the attorney risks losing the ENTIRE case if the "adverse" witness states something which undermines an element of proof which the attorney has so arduously built during his/her DIRECT examination of their own witnesses.
All real questioning of witnesses is done during pre-trial depositions where an attorney is trying to learn what the witness knows, be it good or bad for the attorney's case, and perhaps get an adverse witness to make a statement which might be able to be used during trial.