What are the reasons for ignition timing checking?

What are the reasons for ignition timing checking?
The reason to check the ignition timing, is to get the best efficiency out of the engine. Fuel economy and power.
The spark has to ignite the fuel/air at just the right time, to be efficient.
Most modern engines have this done automatically, by computer and sensors.
You may need to check it after a rebuild, or you suspect problems.

Older engines need to be checked more often, as things wear or bed in.