Whenever a purchase is made, that expenditure goes to income for someone else. If Joe buys an apple that Mary has harvested from her orchard, Joe's expenditure on the apple becomes income for Mary. In more realistic examples, purchases go to income for owners of firms, workers, and the suppliers. Thus, we think of demand as the same thing as income. First, households consume goods.
Firms invest in new factories.
The government spends money as well. Foreigners buy our goods too. She will likely spend a portion of this on, say, an nice meal at a fancy restaurant. The mechanic will get the income, and, in turn, spend. If you are they government, you want to know how much GDP will increase by if, say, you buy a new jet fighter plane.