Fortunately, the authors of the agreement put the reasons behind the agreement into the document... (from Wikipedia, and the agreement)
The aim of the agreement is described in its Article 2, "enhancing the implementation" of the UNFCCC through:
(a) Holding the increase in the global average temperature to well below 2 C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change;
(b) Increasing the ability to adapt to the adverse impacts of climate change and foster climate resilience and low greenhouse gas emissions development, in a manner that does not threaten food production;
(c) Making finance flows consistent with a pathway towards low greenhouse gas emissions and climate-resilient development.