Think about what you are saying. BTDC means before Top Dead Center, so obviously, the spark is ignited 16 degrees before the piston reaches TDC.

The mixture is ignited as it is compressed, so that the timing is perfect between complete combustion and compression. If it ignited too early, it will result in pre-detonation and will kill your motor. If it is ignited too late, you will have an incomplete burn causing wasted fuel and hydrocarbons to be sent out of the combustion chamber. This is why a lot of car fail smog...they need to advance the ignition.