The web site states it’s purpose as:
to specifically thank, credit and acknowledge President George W. Bush for his success in keeping the United States safe from domestic terrorism in the years following the tragic events of 9.11.
At least in the source code that is the stated purpose.
Each visitor may leave their own words of thanks as they relate to his having kept the nation safe during his watch.
The messages of thanks will be presented to the former president as a potential offering for his presidential library.
My message was number 3,7774