Back when men were men and bike frames were steel, we only had 5 cogs on the back. One had to be careful with the selection of front chainrings and rear cogs to make maximum use of every possible combination. That led to some strange shifting sequences. Some guys would type up the various combinations and tape them to their handlebars to help them find the next combination in sequence.
Today, with so many of cogs on the back, that doesn't make as much sense as it used to. Think of your bike as having three gear ranges. Most folks use the middle chainring most of the time and the rear shifter for fine adjustments. Save the granny ring for steep uphills and the big ring for downhills and tailwind days. Your 21 speed bike might only have 10 or 12 ratios that you really use but who cares?