What is the use of the Hi/Lo method in NHibernate?

Hello NHibernate gurus. I have worked with NHibernate for a relavitely short time and have a question.

My understanding is that Hi/Lo is used to generate auto-increment numbers for primary keys. But I cannot think of a real-world situation where you would need this. Why not just rely on the underlying database identity or autonumber facilities?  And, does this not interfere with the database generated autonumbers (Identities)?

Thanks.