Why format? So others like agents and editors can read your work easily, because it’s more professional, because you need to know standard page and word count. The common page format for books is: one inch margins all around, page number in header in the top right hand corner, title of book/last name in the header on the left, double spaced, Times New Roman, 12 pitch font.
Where does one get formatting requirements? Agent and editor web sites. In traditional publishing agent and publisher submission guidelines are on publisher/agent web sites, usually. Always check submission guidelines for the publisher/agent you are targeting to make sure you have the formatting they are requesting. There is nothing more unprofessional then sending in a poorly formatted manuscript. Agents and editors are looking for professionals, for writers who can follow directions and relate on a creative as well as a business level.