Anchor river I would suggest that you ask this question on the engineer's forum and you may get a reply from a powerplant expert.However here is my basic understanding of it as a pilot.By closing the hp you are no longer drawing air off the engine during start.As I mentioned my fcom states that it draws it from the last stage of the compressor.This is obviously the point before the air is then mixed with fuel and ignited.I do not know how much air is drawn through that valve but by closing it I can only assume that you are adding more compressed air into the combustion area and changing the fuel air mix..
Closing the hp valve means air is bled from the Ip stage through the Ip bleed air check valves.The fcom also notes that normally IP air is used as a bleed source to minimise engine pressure losses.I would therefore infer that the HP is closed in order to get the maximum amount of compressed air into the combustion chamber.
I would imagine that through testing it was discovered that for the majority of starts this procedure is not necessary.