What is the use of *-doc packages?

As you noticed, doc packages are not for basic man/info pages. They contain extended documentation, usually in the form of HTML pages. So Generally it's be safe to remove any installed doc packages. just make sure you don't over-remove packages, which is especially easy with *. Additionally regex is somewhat difficult from apt-get, it does not seem to use the standard POSIX/Extended POSIX style. I would actually do something like this.

dpkg -l | awk ' { print $2 } ' | grep '\-doc$'

If you approve of that list displayed, you can then attempt to purge them like this,

sudo apt-get purge `dpkg -l | awk ' { print $2 } ' | grep '\-doc$'`