Back in the day it was the only way to add functionality Apple itself refused to impliment in iOS. App switching, control center, fast toggling of airplane mode and wireless radios. Heck even app folders on the home screen and notifications on the lock screen were originally introduced via Jailbreaking years before Apple finally adopted them. I started Jailbreaking around iOS 5, back when AT&T refused to allow hotspot functionality to those of us on grandfathered unlimited plans. Right up until iOS 9, my phones always had custom font, layouts, carrier text and sounds. I even had blinking Christmas lights wrapped around my lock screen during the holidays. The freedom and flexibility Jailbreaking allowed made the best hardware on the market that much more powerful. Eventually, Apple pulled its head out of its butt, realized users were resorting to Jailbreaking for a valid reason, and started to adopt the best parts of Jailbreaking such as F.Lux which they renamed Night Shift, instead of ostracizing users for wanting to Jailbreak.
Because of that, nowadays there just really isn’t a reason to Jailbreak anymore, which is why it’s fallen out of favor.