A Bible Commentary is useful when you want to know the historical background of what you are reading, what other parts of the bible suggest about a particular passage as well as what history and archeology suggests. Often the point of view and theology of the writer is suggested. It also often provides theological views on the passage, sometimes comparing them. Take a look at
The New Jerome Bible Commentary, which is a well-regarded scholarly bible commentary, published in 1989 as the third edition of the commentary which was brought out in 1968. It is a Catholic work but the views are largely mainstream. It includes nearly verse-by-verse commentary on every book of the bible as well as a wealth of background articles.
The Harpercollins Bible Commentary is a shorter work, thus not so detailed, but readable and quite useful.
The Oxford Companion to the Bible is a an example of a "companion", typically organized as a dictionary rather than with explanations following the bible text chapter by chapter. It has very readable entries on many bible subjects, thus offers an alternate way to dig into Bible background.