When records are added to a table, a copy of the records inserted or modified is stored in a virtual temporary table called "Inserted", which has the same column structure as the actual data table. You can reference this virtual table to examine or filter based on the records that were inserted:
CREATE TRIGGER test ON [Table_1]
set [Field_1] =SUSER_SNAME()
inner join Inserted on [Table_1].[PrimaryKey] = Inserted.[PrimaryKey]
Look up Triggers in Books Online and read about both the Inserted and Deleted virtual tables.
If it's not practically useful, then it's practically useless.