Why should I use Azure Data migration service? Why can't I directly restore the database backup using Azure managed instance?
As the document saied, Azure Database Migration Service integrates some of the functionality of our existing tools and services.
It provides customers with a comprehensive, highly available solution. The service uses the Data Migration Assistant to generate assessment reports that provide recommendations to guide you through the changes required prior to performing a migration.
It's up to you to perform any remediation required. When you're ready to begin the migration process, Azure Database Migration Service performs all of the required steps. You can fire and forget your migration projects with peace of mind, knowing that the process takes advantage of best practices as determined by Microsoft.
DMS is one of the ways to migrate database. You also can directly restore the database backup by using Azure managed instance.