Understanding is just armchair theorizing.

Question: Why is it said that if everyone who studies and practices Buddhism only achieves intellectual understanding (jie wu), the true Buddhadharma will gradually disappear and perish?

Answer: Let me give an example. Suppose everyone buys a book to learn driving skills, or listens to someone explain driving techniques in person, and everyone masters the technical knowledge, all believing they can drive. Yet they have never actually driven a car, never even touched a steering wheel. Then these people go on to write books and teach others how to drive. In that case, what would driving skills become after being passed down like this? Would such knowledge still have any practical, hands-on value? Would anyone still drive in the future? Would there still be anyone who knows how to drive? After this, wouldn't the skill of driving be lost, degenerating into a mere armchair game of paper talk?

The Buddhadharma is the same. If everyone only has intellectual understanding without real realization, and it is passed from one to another, then in the end, even those with intellectual understanding will cease to exist, and even the intellectually understood Buddhadharma will become distorted, and the true Buddhadharma will be lost. This is a situation the Buddha could not bear to see, and as the Buddha's true disciples, we would also feel deep pain over it.

Comprehension and intellectual understanding are essentially different. Intellectual understanding is something like guessing the general way the tathāgatagarbha functions; it is not observed through direct perception, and is not as clear as seeing an object right before one's eyes. What is merely guessed at leaves the manas still unenlightened; the mind remains unsteady, without a firm footing, and is easily swayed. In such a state, one cannot observe the specific operations of the tathāgatagarbha, and cannot give birth to genuine great wisdom. Ordinary comprehension and understanding means one has learned and grasped the knowledge fairly well, but has not yet entered the stage of actual deliberation and contemplative practice (vipaśyanā-caryā) and investigation; one still has not the slightest awareness of the operation of the tathāgatagarbha. This is still far from the level of intellectual understanding, and the gap from realized enlightenment is even greater.