A lot will have to happen _during_ lockdown, you can't just sit idly by. Basically it will buy time.
Of course the virus will still spread once lockdown is lifted, but for countries like the US (not even to mention Italy and Spain) who started behind the curve any time you can buy to bolster your response should be taken.
3blue1brown's latest video on YouTube demonstrates that we can also buy time by using a combination of mitigation techniques short of a full lockdown. It's a video of a computer simulation created by a mathematician fwiw, but it shows some of our options, and that brute force may not be required.
Sometimes I want to advocate for a full global lockdown, but I think about the random emergency appendectomy, and other weird edge cases that would be complicated by a lockdown of any magnitude.
Of course the virus will still spread once lockdown is lifted, but for countries like the US (not even to mention Italy and Spain) who started behind the curve any time you can buy to bolster your response should be taken.