Deficit spending in a keynesian model is only supposed to occur during economic slowdowns. The idea is that its not the amount of money, but the movement of it, which makes an economy improve. Governments have a strong ability to get money circulating and flowing to "prime the pump" of the rest of the economy. I focus generally on American policy (maybe that makes me a dickhead American, sorry?) but in the U.S., the government is the largest buyer of goods. When they cut spending, the market loses its biggest customer, when they spend at a deficit, its like the biggest customer is on a shopping spree. The idea is that money starts going into business, businesses make more money, spend more money, and keep the fluidity of money going, strengthening the economy. Also, strengthened businesses end up paying more in taxes when they do more business. Remember: the government isn't burning that money, they are sending it out into the economy.
On the other hand, austerity reduces spending by the biggest buyer of goods, makes companies save more and spend less, and slows the economy down. That's why Europe is fucked, and why the U.S. sunk deeper into the great depression when Hoover tried to shrink the budget. The governments are essentially sucking money out of the economy and not putting it back in. The U.S. is going to be the next one to prove this is the case