Imagine that you work in a team of other developers, and everyone is busy working, making changes to the code and the database structure. Keeping up with all of those changes to a database can become a challenge, particularly if many people are working on roughly the same area of the project.
CodeIgniter Migration gives you the option to install (or rollback) the changes of a database structure that might support the changes in the code. For example, if you were working on coding changes for, say, a user registration script—this change requires a column to be added to a database table; you could include a CodeIgniter database migration script with your version control commit (assuming you're using version control)—other developers will now know that for your code change to work they must run the migration which would amend their database.
Migration also allows you to roll back changes. This should not be confused with the database concept of rolling back...