I read some article about Data-tier Application, but i don't understand when we should or better to use it, anyone can give me an example about it?
One of the reasons of using Data-tier Application is described here
You can help your development team manage changes to databases and database servers by creating a DAC project, database project, or a server project and putting it under version control. Members of your team can then check out files to make, build, and test changes in an isolated development environment, or sandbox, before sharing them with the team. To help ensure code quality, your team can finish and test all changes for a particular release of the database in a staging environment before you deploy the changes into production.