How Record.GET function works?

I everyone, am new to MS Dynamics NAV. 

I want to know when we ll use Record.GET function and how it works..? 

I mean it works only for setup tables, because in setup tables, we ll empty the primary key field, since only one record in setup tables.but it won't works for other type tables, for example master tables... even it works for other tables also, only in case of if table contains record with primary key fields are empty in it. So we can fetch only this record. because we can't insert more than one record with empty primary key fields..?

I just want to know, when, what and how to use this function???

If u didn't understand pls replay me.

Thank you