BITD 40+years ago, most entry- to mid-level road bikes had a 52/42t crankset up front and a 14-28t five-speed freewheel out back. Generally a wide-enough gearing for most of the country.
'Race bikes' would drop the rear gears down to 14-21 or less.
Triple cranksets appeared with smaller inner tooth counts for hill-climbing ability.
Then along came six speed freewheels that allowed for more gears in a person's preferred range for varying conditions from 13t up to 35t..
Then seven, eight, nine, ten-speed rear clusters with small rings of down to 11t.
Along with that latest rear cluster 'advancement' came the 'compact crankset' where the big rings got smaller, so the overall gear ratio stayed the same...
The goal is to get gear ratios in the range you need. Use this tool
https://www.gear-calculator.com to figure effective gear ratios... Use this app to change the front chainrings or rear sprockets and play around!