why is trigger used in sql code example

Example 1: how use trigger in sql

CREATE TRIGGER Product_Details_tr 
BEFORE INSERT ON Product_Details 
FOR EACH ROW 
SET NEW.User_ID = CURRENT_USER();

Example 2: SQL trigger

CREATE TRIGGER [schema_name.]trigger_name
ON table_name
{FOR | AFTER | INSTEAD OF} {[INSERT] [,] [UPDATE] [,] [DELETE]}
AS
{sql_statements}