When you want to view (or display) all possible combinations. It doesn't make much sense for example with relations that describe things that happened or are (

When you want to view (or display) all possible combinations. It doesn't make much sense for example with relations that describe things that happened or are (facts). For example you wouldn't cross join a users and addresses table. But when relations are composable, say a product with selectable features, then it can make sense to cross join. An example: You ask a user a few questions and want to suggest a solution or product based on the combinations of the answers. The suggestions are provided by domain experts in advance. In this case you might want to use a cross join to provide a matrix of all combinations to your domain experts. They can fill in valid suggestions for some and invalid ones for others. Based on their input you can now automatically generate a questionnaire.