The data tier contains the database. It might also include data access procedures. The database can be placed in a single location or it can be distributed, if required. The key 3-tier benefit is improved scalability since the application servers can be deployed on many machines. Since the client does not have direct access to the database, it is more difficult for a client to obtain unauthorized data.
Business logic is generally more secure since it is placed on a more secure central server.
Changes to business logic only need to be updated on the application servers and do not have to be distributed to all the clients. With redundant servers, it is possible to architect an application so that it can recover from network or server failures. It is likely that additional programming effort will be required to manage tasks that an automated tool might handle in a 2-tier environment. The complexity concerns in the 3-tier model cannot be easily dismissed. In many environments, certain benefits of the 3-tier model are required.