Do reviewers of research proposals for a fellowship read the whole application, or only the scientific part? If you are applying for some fellowships, there are many fields which you have to fill in. For example, what will be benefits for a host country, what will be the benefit for your home country, what will be the benefit for you or for your host supervisor, what will you learn, what have you learned yet, etc. There can be a scientific abstract, lay abstract or questions like what will be the impact of a fellowship on your future.

Who will read your whole application? 

Scientific reviewers of your project will get only your research proposal, or is it their role to read all this fields?  Are these field kept just for cases, when there are two similar candidates to get support, and than these fields can be critical?
From my experience, reviewers get the whole proposal and also read the whole proposal. Reviewers try to get a complete picture of the applicant and skim the proposal for all relevant information in this regard. This means, that probably no reviewer will read every word of the proposal, but potentially, every word will be read by some reviewer. The reviewers base their review on the whole picture they get, but usually their feedback is only for the part for which they are specialists (e.g. the scientific strength of the applicant and so on…).