Why is often necessary to follow extensive purification procedures in the isolation of organic compounds?

To remove impurities

Explanation:

There are different substances present in  an organic compound. If an organic compound contains different substances then there are some physical or chemical methods for the process of isolation, so that we will get different substances from the organic compound, which may or may not contain impurities.To get rid of impurities after the isolation process purification process has to be done, so that we will get a purified substance from an organic compound.Purification may be done using the method of sublimation, distillation and so on.

well there was still gas in the can

Explanation:  so by putting it in the incinerator the gas in the can got hotter and hotter etc.  and you got yourself your thermal reaction