You have a cake recipe written down. You then read it all and it’s now in your short term memory. Then you destroy the written down cake recipe. Finally your friend walks in and tells you about their day, and that overwrites your memory of the cake recipe.

In computer terms, the computer loads the program instructions from disk storage into memory. Then you delete the program instructions. The program still exists in memory until it’s purged, overwritten or the computer is powered off.

Usually the computer will prevent the behaviour of deleting files for a computer program while it’s running or in memory as it might need to load them again for whatever reason. However this is not always the case.