I discovered this chart today. It was reassuring to learn that my 54 cm carbon frame is the correct size. I am 5′ 9″ with a 30″ inseam. I never doubted it. The bike felt correct from day one. 30 years ago, I purchased a custom 21″ bespoke English touring frame. A 54 cm frame. I know what the right size feels like.
Not everyone purchases bikes from a shop or is sized in advance. Many purchase used bikes or shop on-line for the best deal. Unfortunately, the best deal may not be the best fit. I know one such individual who got a great deal on a bike sight unseen and he is now struggling to make it fit.
There are many things to consider when purchasing a bike but at least know your correct frame size. The chart above is a good start.