The OpenGL library is composed of elementary functions to do elementary tasks (select a texture, draw a triangle strip, setup a light, etc...). The aim of a library is to increase one programmer productivity by simplifying repetitive tasks.
e.g: you need to setup a 3D scene with lighting and display objects. A basic library will provide you a simple interface to allow you to setup your scene with a minimum number of function calls. When you develop a lot of 3D applications, you do not want to do the same repetitive tasks and you do not necessarily have time to develop a library simplifying all those repetitive tasks (especially in the industry). This is why you use a Graphic library.

Of course using a library does take time to learn and master. Some functions necessitate to have an understanding of the underlying rendering solution. Before using a library at its full potential, you need to have a basic understanding of the OpenGL rendering system. If you are not a beginner and work in the industry, you have to code to provide quickly a working demo. Thus if you aim to work in the industry, you will need to learn the basics of OpenGL, then learn to use a library to increase your own productivity.

Instead of simply using a rendering library, you could use an engine to increase your own productivity. The definition of an engine is not set in stone. I propose my own interpretation which may differ from somebody else. To my mind engine functionnalities are broader than simply providing a simple interface to rendering 3D scenes: the functionnalities also encompass resource management, user I/O, memory management, sound rendering. The engine is a set of common functionnalities geared towards a certain type of game like FPS or RTS. The rendering library is but a part of the engine.

I hope I answered all your questions.
Ghostly yours,
Red.