Increase in the numbers is showing that testing is still catching up with the real number of infected out there. For a population the size of the US, considering the delayed response in virtually every aspect, having only 20-30K officially infected is actually worrisome as it is indicative of inadequate testing procedures and availability. It is indicative of the government not having things under control as they likely lack idea of the real scope of the spread.
Having a good estimate of the number of infected, would allow for a good estimate of hospital beds and supplies that will be necessary in the coming days and months.
Also, a good estimate of the scale of infection would be helpful in taking more timely and better, more adequate, measures in terms of stopping the spread from growing.
For example, say you have a US state with only 50 official cases, that does't sound like a dire situation, and the governor of said state probably won't take sever lockdown measures. But what if tests were widely available and testing showed that the (projected) number of infected is not 50 but 50K. Having that information would result in more timely measures to limit the spread of the virus.