Why did they come up with, if you can throw out a dump?

I feel that the question is the same ^^ Sneakers do not throw.

Answer 1, Authority 100%

Most often, migration is used to properly cross the database to the new structure. In projects, it often happens that everything was done in one structure before, and then the existing database should be remade, but it is impossible to lose these loss. To do this, use migration.

Answer 2, Authority 50%

migration , in this context, – change the database structure from one version to another (usually more new).

Therefore, in cases where the context is not obvious, use a more accurate term: Version Migration Databases .

why do you need?

Database version must match the version of the application. These are inseparable parts of one whole. This means that when deliverying a new version of the application in production, you must update the database version, otherwise the application simply will not be able to work correctly.

Here is a good full answer, which will give food for reflection:
Version Migration Database Structures: Basic Approaches