What is this knob for? When I open it I thought it’s to stop the blade from over-opening. But it doesn’t touch anything. So there must be some other use for it.
It's there as something with which to pull the blade open. Many knives have a crescent-shaped indentation for this purpose (which is cheaper to manufacture); the advantage of the knob is that usually it's possible to use it one-handed. It helps if you're right-handed, as it's easiest pushed with one's right thumb.
It's to open the knife:
- grab the knife with the right hand (even if not dominant)
- move your fingers out of the way of the blade by placing them on the clip on the back side
- use thumb to flip the blade open; this can be done slowly and silently or rapidly.
It's rooted in armed martial arts but once the motion is perfected, it becomes Your default forever. Saves 1-2 seconds every time you open a switchblade(what the thing in the picture is called).
Do note that this kind of knife is illegal is some jurisdictions e.g. Germany. Because it is "a knife you can open with one hand". Not too illegal but don't wave it around.
Another tangential point to mention is maintenance. After using it for any hard work do the following. Open it up and rock the blade sideways. If it moves, you need to tighten the big central screw. Not too much; you'll get a feel. The small screws also come lose but less often.
If you want to destroy it, use it for knife-throwing: I give it 5 hits as it's somewhat decent.