Triple drivetrains
Back in my day a 21 speed, or 3x7 bike was the standard for top end bikes. Now days they have been phased out for 2x or 1x drivetrains.
Why triple?
Triple drivetrains are still useful for a few things.
Touring
Hauling bike packs up hill takes some gears, and on long rides, you want a lot of options so you can keep spinning.
Mountain
Sometimes dropping a ring in front to go up a hill is faster than shifting down a few gears in back.