Intervals in granularitySpec

When we define a data schema , found that there is something called interval in granularityspec, not sure why it  is needed,
as per my understanding, we define schema once for batch ingestion and continuously add the data.  if we need to put the intervals every time
we ingest new data. can someone here would be able to help.

  "granularitySpec" : {
    "segmentGranularity" : "DAY",
    "queryGranularity" : "NONE",
    "intervals" : [ "2013-08-31/2013-09-01" ]