I would classify this as a "great to know but not in any way essential" for someone, so I am curious why you actually take a chart with you? How do you use it?

I would classify this as a "great to know but not in any way essential" for someone, so I am curious why you actually take a chart with you? How do you use it? I can understand using a chart to plan things out ahead of time, though to me it seems that it is easy to figure out workable gears when starting out just by riding and eventually you know how to shift in a given situation. The more serious riders will refine that by trying out different combinations in more controlled experiments (power meters when you get to Serious Training). The best gearing always depends on the rider.