The use case is if you have several customers or users who will upload and download data. If you set limits on the project level only, each user could consume the full limits. But this might not be what you want. You might want to allocate fixed allowance to each user that he will be able to consume. This would be easy if you could create a bucket, set limits on the bucket and let the user use it.
Other way would be what has been already in the question mentioned here: Storage Limits, Sub-users, etc with sub-users. With this you would be able to create a user or usergroup and assign the limits to it.
Maybe even limits assigned to specific access grants would be doable?
This would give much more and fine-grained control over allowed resource consumption because I think it is not really feasible to create 1000 or more projects…