Adjustments were made for several reasons, primarily because measurements prior to 1940 were often made at warmer parts of the day.
Total adjustment was about 0.3C; the US has warmed by more than that since 1981. I could see how someone could claim this adjustment invalidates the warming narrative in say the 1990s when the adjustment made was large relative to the warming trend, but at this point the data supports a warming trend even if you don't allow the adjustments.
For example, all of the past 10 years have been warmer than any year in the 1930s and 1940s even using the unadjusted data (contrary to your assertion in https://news.ycombinator.com/item?id=30727107).
