What's the point in specifying the grant number in a paper's acknowledgments? I understand why people mention the grant agency, but what's the point in specifying the grant number in a paper's acknowledgments?

Examples:


  The authors acknowledge
  the support of the U.S. Army Research Office under
  grant number W911NF-10-1-0533
  
  This work was supported
  by the National Key Basic Research Program of
  China via grant 2014CB340503


I am mostly interested in the field of computer science > machine learning, and English-speaking venues.
The agencies that provide this funding often require such acknowledgement. NSF does. The format, more or less, is up to the agency. I don't know why they do it, but I believe they want readers to be able to find the programs that fund the work so that other potential applicant can find future versions of the program and possibly apply.

Edit: Look at Part J of NSF's grants guide for their format