JPA - @Column (unique=true) - What is really point of having 'unique' attribute?

unique in @Column is used only if you let your JPA provider create the database for you - it will create the unique constraint on the specified column. But if you already have the database, or you alter it once created, then unique doesn't have any effect.

unique=true in @Column annotation will be used only in DDL generation, it doesn't have any impact during runtime. The actual uniqueness checks happens in the database.