Leave this setting enabled for two reasons.
The first is that it does what it says on the tin -- it automatically controls brightness. This means your iPhone will cleverly dim the display when you're in a dark room and brighten it up when it's a sunny day when you might not otherwise be able to see the display at all.
Keeping this on is also a great battery saver, especially for people who otherwise would keep their display at full brightness all the time.
Oh, and something to bear in mind about keeping your display on full brightness is that it can, over time, damage your display. It's far better to allow your iPhone to control this setting.
Settings > Accessibility > Display & Text Size and make sure Auto-Brightness is enabled.