File: migration.rdoc [Sequel: The Database Toolkit for Ruby]:
Migrations are optional, you don't have to use them. You can always just create the necessary database structure manually using Sequel's schema modification methods or another database tool. However, if you are dealing with other developers, you'll have to send them all of the changes you are making. Even if you aren't dealing with other developers, you generally have to make the schema changes in 3 places (development, testing, and production), and it's probably easier to use the migrations system to apply the schema changes than it is to keep track of the changes manually and execute them manually at the appropriate time.