Main argumentation in introductory chapter of Phd thesis?
Every dissertation or thesis should have an introduction. 

When I receive a student's thesis to read, I usually do not have numerous hours to pick out the details of every page. Having a 4-8 page introduction is extremely helpful in allowing me to learn what is most important in the overall arguments of the thesis. An introduction allows readers to see the major theme of the work.

When I wrote my own dissertation, I treated the introduction chapter much like an extended abstract. It highlighted what the reader needed to watch for and established the overall theme of my work. 

The introduction can also serve a useful purpose of providing a place to take care of some of the administrative aspects of the thesis such as acknowledgements and holistic citations ("This thesis was written using LaTeX; all code was written in C++). These things are given their own pages at times, but I found it useful to be able to just place them in a paragraph in the introduction.