An OBD reader is one of those tools whose usefulness depends heavily on expectations. If someone expects a small gadget to tell them exactly which part to replace, disappointment comes quickly. But if it is treated as a clue-giving tool, it can be genuinely useful.
The most practical use is reading fault codes when an engine or system warning appears on the dashboard. A code is not always a final diagnosis, but it can help you understand whether the issue points toward ignition, mixture, a sensor or something else.
It is also valuable because you can write the code down before someone clears it. That way important information is not lost when you later ask a workshop or a forum for help. A precise code plus symptom description is a much better starting point than saying only that some warning light is on.
At the same time, it is important not to assume that the code always means the part named on the screen is definitely bad. A faulty sensor, poor connection or another supporting issue can easily trigger a code that points in the wrong direction.
A simple OBD reader is especially helpful if you have multiple cars, an older car or simply want to understand your vehicle better. It does not replace a good workshop, but it can stop you from being completely in the dark.
If you decide to buy one, choose something simple and reliable rather than chasing the most exotic promises. The real value comes less from the number of features and more from using the information calmly and sensibly.