Get the units wrong and you either strip the fastener or leave it dangerously loose - genuinely worth getting right, not eyeballed. Real fastener presets included.
Under-torque a lug nut and it can work loose over time, letting a wheel shift on the hub. Over-torque it and you stretch the studs and can warp the brake rotors - both are real failure modes, not theoretical ones. A single mixed-up unit (reading a "90" as ft-lb when the manual meant Nm, for instance) is roughly a 36% error in the wrong direction, which is exactly the kind of mistake that's easy to make and expensive to have made.
Where the spark plug preset comes from - and why your manual may say less. The 18 ft-lb preset is the bottom of the range NGK publishes for a 14mm gasketed plug in an aluminum head (18-21.6 ft-lb, per NGK's own installation guidance). But vehicle manufacturers frequently specify lower - Honda and Ford manuals commonly say 13 ft-lb for the same plug size. That's not a contradiction to shrug at: the plug maker rates what the plug can withstand; the engine maker owns the threads in the head. When they differ, your engine manual wins. Most torque charts print a single number and skip this entirely.
The bike stem preset uses the 4-6 Nm typical range for threadless stem pinch bolts published by Park Tool, taking 5 Nm as the working value. Two overrides beat any chart, in order: the number etched next to the bolt on your actual stem, and lower figures for carbon parts - carbon bars crush before they strip.