When you first start writing on a new topic you are reviewing, the writing process is a stage of learning. As you get stuff down onto paper, you start to be able to see how the parts come together and start to actually understand and form your own ideas and opinions. At this stage when anyone else reads your work (eg your supervisor) it may be hard for them to follow and your own ideas may not be clearly articulated.
Once you have learned enough (through writing about it) to really get to grips with the material and work out your own opinions and what you are actually saying, you can begin to restructure your work to present your opinion and present the evidence and arguments that support it. This stage usually requires a major restructure of the work .
Later, once you have this restructuring done, you need to revise again to produce a draft that another person can follow. Part of this process involves effective signposting. In her blogpost Katherine Firth provides the best explanation of this that I have come across with some really helpful examples. A really worthwhile read.
Effective Signposting, Katherine Firth, Research Degree Voodoo