Just look at what almost one degree Celsius of warming has done: more extreme heat and wildfires, bigger storms and droughts, hotter and higher seas, warmer summers and winters. Many animals and plants are shifting to new areas, seeking familiar ecosystems elsewhere. Industries and economies are either facing or preparing for impacts.
This number matters because every small fraction of a degree matters. The Paris Agreement in 2015 set a worldwide aspirational target to hold warming to 1.5°C, down from a 2°C target. Staying below 1.5°C would prevent hundreds of millions of people from suffering through extreme heat waves and give other living things more time to adjust to changing conditions.