ALTER TABLE CHECK CONSTRAINT
It ensures that the constraint is enabled after it is created.  Your ALTER TABLE statement includes WITH NOCHECK which is the piece that says not to check for existing bad data during the creation of the constraint.

As written, the existing data will not be checked against the constraint because of the WITH NOCHECK in the first statement.  Issuing the second statement will enable the check against the constraint for any future changes to the table that are covered by the constraint, up to the point that an ALTER TABLE [dbo].[T2] NOCHECK CONSTRAINT [FK_T2_T1] is issued.

The statements, as written, are basically saying "Create this foreign key constraint but don't check it against existing data.  Make it active for any upcoming changes to the data."