when nobody spends money, the economy goes to shit. the government borrowed from the future to prevent huge reduction in the economy so that, hopefully, the averted disaster will allow a spring towards economic growth that can compensate for the debt taken out now.
honestly, if the government hadn't acted, the ecnomy would have tanked so horrendously since the fear of financial meltdown was reinforcing itself. the government was the only thing that pulled the economy away from the abyss.