Install OpenCV from source or via Pip?
OpenCV is always under development, and the thing is some parts of the library is not going to published, due to compatibility and copyright issues, but if you use the source then you can have all the capabilities that you need. SURF & SIFT are examples of this problem.
I've seen 2 ways of installing OpenCV (there might be more ways which I don't know): 

Installing from the source
Installing with pip: pip install opencv-python

My question is, why we need to install OpenCV from the source while we can simply install it using pip? Since people are using both of them, both must be useful. If so, there are any conditions for selecting one of them?