My work often involves working on one of many products/deliverables for a client. It would be much easier for me and my clients to organize our teams based on these deliverables by being able to group several teams together.
For example, the group “Product A” could have its own “Engineering” and “Marketing” teams, then each team can have their own projects as usual. Currently the best we can do is to create a single team like “Client Name - Product A” and have each initiative/milestone it’s own project.
Note that this problem still wouldn’t be solved if my clients had their own organization because at the end of the day we’re still working on several deliverables/products with their own teams.