Could be anything. Suppose if a client wants to refer an bug to a number they use for changes, they'd like to have a custom numeric field. Or they have their own definition of statusses which are used throughout their company. For example 'Accepted' rather than 'Confirmed'.

Responding on the why part of the question, over the years I've noticed that it does not work to force users of bug processing software to use a fixed amount of statussen. Most companies have already developed some kind of process and do not want to change that. If switching to Damnbugs means that these companies must change their working process, they'll never adopt the new system.