Sometimes deciding what the bug is depends on knowing what people are doing.

For example, lets say the report is that a program crashes when a particular set of options is used together. The developers thought that there were no situations where it made sense for someone to use that particular set of options.

They need to decide if the bug is that the program accepts that combination of options rather than giving an error about an illegal option combination, or the bug is that the developers were wrong and that combination should be made to work.