Answer:
The network database model implements application logic in the data structure that limits the flexibility. In the network model, there is always the "first" in the table, and the "last" in the table. If you read the table, you always read it the same way. The database allows you to implement "joins" as sets - and they are always ready. The "union" can be empty, it can have a first and last member and members can refer to other tables. If the systems requires a number on the invoice, you cannot store the invoice without giving it a number.
But then, if the application logic changes you need to restructure the database and add indexes that the users can use to improve the new logic. If you add a field, you have to restructure the entire table (or "realm").
It is fast when used as intended, it is easy to write applications for, but is not intended for new use.