If the CPI is imperfect why do we use it?

Answer 1

Its imperfect because not everyone uses the same basket of goods, and its imperfect because the basket of goods doesn't change that often. However, it does tell us overall at what rate are prices increasing in the US. A better measure called the PPI is available but it takes longer for it to be calculated

Answer 2

No economic statistic is perfect -- not a single one of them. Government statisticians cannot ever know what is REALLY happening, they can only make estimates. The CPI calculations could use some improvements, and I think it substantially overestimates inflation during times of rapidly changing consumption patterns, but it's better than nothing.

To the above, the PPI is certainly not better than the CPI, and is quite inappropriate as a measure of general inflation faced by the public.