Gearing can be a very involved subject and requires understanding of cog ratios, wheel circumference, pedalling speed (cadence) and road speed.
Bike manufacturers tend to give a very wide gear range to cover a wide range of cycling terrain, speed and rider strength.
In general, experienced riders try to fine tune their bike's gearing to match their needs, by swapping chainrings and cogs. Before doing this, you should plan your requirements for terrain, speed, comfort, duration of ride, wind, etc etc.
At the moment I'm doing short triathlon and criterium type club racing, on relatively level roads, with light winds. So I don't need a lot of range in my setup. I'm riding a 25 year old bike with 7 speeds on the rear (that's plenty for my use). I have 49 Teeth and 42 Teeth on the front chainrings (yes only 2), and 13,14,15,16,17,18,19 on the rear. This gives me a very close selection of gears so I can keep an optimal cadence regardless of the road conditions, within the expected racing speed range of 24 to 52 km/h.
In the usual race speed range of 30 to 40 km/h I have a perfect gear to choose from, for almost every 1km/h increment.
BUT............ I would be hopelessly lost in hilly terrain, as I don't have gears for climbing or decending.
So if I go away to a race in a hilly area I change my front rings to 52/42 Teeth, and change my rear wheel to 12,13,14,16,18,21,24.
The finer points of choosing exactly which cogs to use, requires careful study of a gear chart and considerable experience. Even then, there will always be arguments about gearing.
In my experience, most owners of 18, 21, 24 or 27 speed bikes only have 5 or 6 gears in the most used speed range. Most of the gears are wasted. That's because the manudacturer can't predict your exact usage, so they give you more than you need.
To get the most out of your bike you can get rid of a lot of gears, and tune the rest to your needs.
So, sort out what type of riding you want to do, and ask an expert to design a system for you.
Bobthe....