https://www.bbc.com/news/health-52473523Is it the most important number?
The reproduction number is one of the big three.
Another is severity - if you have a very mild disease that does not cause many problems then you can relax a bit. Coronavirus, and the disease it causes, Covid-19, is severe and deadly, unfortunately.
The last is the number of cases, which is important for deciding when to act. If you have a high number, but ease restrictions so the reproduction number is about one then you will continue to have a high number of cases.
[My editorial comment is that the USA is still producing about 200k cases per week, though slowly, slowly going lower. In other words, the USA generates more cases than the entire UK ever has, every week.]