In the old days, if you had fuel, spark and you were somewhere close to correct in your ignition timing, your car’s engine would run.
Modern computer-controlled vehicles also must have the proper mixture of air and fuel ignited at the proper instant in order to operate. The difference today is that a variety of engine parameters must be controlled.
Consequently this is to ensure highly efficient combustion for better fuel economy and reduced exhaust emissions. Hence the reality is that it has become very difficult to diagnose a vehicle problem.