Usage of 在 in Place complement, Time Complement
when to use and omit 在 in Place complement, Time Complement?

when a sentence need to let other person fully understand or the subject is important or 
 in a sentence.

(在)包里，他拿了一本书。 在 can also omit.


It can express as 

他拿了一本(在)包里的书。


Place

我在家。


Here 在 is a important word to tell other information about where you are.