In general, building programs interactively by changing them as they run. That's the default, normal way to write programs in certain languages (notably in Comm

In general, building programs interactively by changing them as they run. That's the default, normal way to write programs in certain languages (notably in Common Lisp and its antecedents and in Smalltalk). In such languages, you want to be able to use the entire language at any arbitrary point, including at a point where execution has halted for the moment because of some error that has occurred in some arbitrary place. The condition system provides a nice set of tools for doing that.