Manual install vs conda install tensorflow-gpu

Question

Asking mainly for Ubuntu 16.04/18.04. What's the difference between installing CUDA and cuDNN together with tensorflow-gpu in conda (conda install tensorflow-gpu), and installing it all by hand and then using pip? Does it mean CUDA and cuDNN are only available in the environment where conda install tensorflow-gpu was invoked? Also, why do some people use the -c flag when installing it?