ending to the percentage of that country's GDP when GDP is not how much money the government has, but how much people spend each year in that country?...

Why do people usually compare government spending to the percentage of that country's GDP when GDP is not how much money the government has, but how much people spend each year in that country? (For example, people like to say US defence budget per year is xx% of US annual GDP) How is this then a good representation of how the government distribute total money it has to each department?