It forces you to think on subjects and ideas you probably wouldn't have on your own. For some it helps people see the philosophical ideas that many have pondered on since the beginning of time ( love, death, friendship, bravery, ect.)
Communication, learning, entertainment, information/direction (i.e. traffic signs).
Mechanical. The process fires up synapses in your brain that would remain dormant otherwise.
Communication. Without words, written language or reading the transferring of thoughts/ideas would be limited to verbal or other methods and the world would be a less efficient place.
Enjoyment, relaxation, escapism, education, helps develop language skills.
Because osmosis really won't work with books
Do you need one ?
It's entertainment for me, as well as something to help pass time. I find it more entertaining than watching a movie.