What is the implication at attempting an attach without detach?

Sql server attach prerequisite states that a db must be detached before trying to attach it to another instance (https://learn.microsoft.com/en-us/sql/relational-databases/databases/attach-a-database?view=sql-server-ver15#Prerequisites) However this migration link (https://learn.microsoft.com/en-us/sql/database-engine/install-windows/choose-a-database-engine-upgrade-method?view=sql-server-ver15#migrate-to-a-new-installation) recommends an attach without any mention about detach. Example: What is the implication at attempting an attach without detach?