You can gather all issues using the Product module. This enables you to get a gathered view of what is happening with a product or service, regardless of how many different projects are involved. For better organization view, the products can be arranged in different folders.
This enables you to get a good multi-project overview of your issues. This also enables you to have releases, etc specified in a multi-project view. An example where this is good is if you wish to see all development tasks, support issues, research, etc that have lead to a new release. These are usually gathered in different projects.