On large systems with many users, you often need to control the amount of disk space a user can use. Disk quotas are designed specifically for this purpose. Quotas, managed per partition, can be set for both individual users and for groups; quotas for the group need not be as large as the aggregate quotas for the individuals in the groups.
When files are created, both a user and a group own them; ownership of the files is always part of the metadata about the files. This makes quotas based on both users and groups easy to manage.
Disk quota management is not really useful or needed on a home system and rarely, if ever, on a small office system. It is unlikely you will see or implement this in either circumstance.
To manage ...