What's the point in specifying the grant number in a paper's acknowledgments?
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