Sentence:  The idea of having a time lag is to allow for situations when a <WSD>corruption</WSD> of some type affects the source server. If a corruption occurs, you do not want it to replicate to the copy of the database, so the time lag gives administrators the opportunity to recognize that a problem exists and then to have the ability to switch from the database copy if the <WSD>corruption</WSD> is so bad that it renders the original database unusable. 
Meaning of the word marked with <WSD> tags:  The destruction of data by manipulation of parts of it, either by deliberate or accidental human action or by imperfections in storage or transmission media.