Why do we use 45 degree diagram in economics?

Thanks to invitation; A 45-degree line is used both as a guideline that provides insight into the measured variables and as a critical part of the analysis. This line is perhaps most important in Keynesian economics. It is used in a diagram measuring consumption and income as a means of deriving saving---the difference between income and consumption. It is also used in the Keynesian model (or Keynesian cross) diagram measuring aggregate expenditures and aggregate production as a means of identifying equilibrium--the equality between aggregate expenditures and aggregate production.