One of Apple’s new features in iOS 8 is the QuickType keyboard, where the system will analyze your entered text to offer three suggestions of possible words as you type them. This can be used to assemble some amusing sentences if you are in for a laugh or two, but can also be quite useful when typing with one hand. However, the keyboard does take up screen real estate and there are times when entering text will leave you with precious few lines to see for composing whatever it is you are typing.
Of course, iPad owners have the option of using the split keyboard that can reveal more of your screen, but iPhones do not have this feature, and especially on the smaller screen will have more real estate taken up by QuickType.
When the keyboard is hidden, it should remain so until you purposefully reveal it again, so while there are no settings in iOS to disable the keyboard, you can use this feature to effectively do so if you do not use the keyboard.