The field product model is used by some stores as a unique identifier to track items in the store. Some vendors call this a SKU or stock keeping unit.
Using a model number is completely optional, and won’t make sense for some stores.
If you do use model numbers, you may wish to ensure that they are unique. The plugin Find Duplicate Models may be used for this. This process is very important for stores that use Quickbooks or other external systems that require unique model identifiers.