Apple has a reputation for leaving out features and buttons on iPhone that are deemed unnecessary. The original model left out a physical keyboard which was an industry standard at the time, and since then, they’ve removed its headphone jack, and even its home button. But one thing the iPhone still has is its silent switch! Something many people don’t use, especially those switching from Android, where silent mode is activated with a software button instead.
Well, the story behind the silent switch goes back to the iPod. It had a nearly identical switch button that turned orange when activated, just like on iPhone. Except it was used for hold mode. A feature that deactivated the iPod’s buttons to prevent unintended presses while in a pocket or bag.
Apple carried this hardware switch over to the iPhone although the touchscreen device didn’t need a hold mode. Since the new ‘swipe to unlock’ action already prevented accidental inputs. Instead, Apple used the switch for silent mode. Which was quite unusual at the time. Typically, cellphones could be placed on mute by holding the volume down button until the device vibrated. Confirming silent mode was activated. But Apple recognized the confusion this could cause on a more advanced smartphone.
Turning down the volume would not only silence the ringer and notification sounds but also prevent sound from being played while watching Youtube videos or playing a song. Requiring users to continuously adjust their volume back and forth. A more convenient solution was adding a silent switch that would mute all sounds while the device was asleep, but allow audio to pass through while the device was being used for other tasks. And while Android has a similar solution, its silent button is software-based, located in a drop-down menu.
This could be problematic in certain situations like in a movie theater, during a meeting, or in bed. Since you’d have to wake the display and interact with the device. Something that could be seen as rude, inconvenient, or at the very least uncomfortable. So by adding a physical switch, iPhone users can turn on the silent mode by simply reaching into their pocket. Or confirm silent mode is already on by glancing at the switch and ensuring the orange line is visible.
All of this prevents you from waking up the device which could be disruptive.
Source: Why iPhones Have A Silent Switch