The portrait lock is one of the most useful and energy consuming options. Keeping it always on is not recommended. However, it is often simply necessary.
Its main purpose is to prevent the phone from changing the position of the screen. In other words, if the device turns to one side, the screen content will remain in a vertical form if the option is enabled. When it is inactive, the screen, following the device, will fall to one side.
Screen position when orientation is locked
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Screen position when orientation is unlocked
Most often, the function has to be disabled in cases when it is convenient for the user to type the test with two hands, when watching videos specifically on the device (third-party applications themselves turn the screen) and to get more functionality of the calculator. Few people know about the last case, but it is very useful for schoolchildren, students and accountants. When the screen is rotated, the calculator turns from ordinary to engineering.
Regular calculator
Engineering calculator with orientation unlocked
The option, as mentioned earlier, is useful but wastes a significant amount of battery power. Try to keep the orientation always locked when not needed.