The sketch book is usually carried with the animator when travelling and moving to regular private places ( means some places the animator goes and sit down to relax or watch something happening around ).
It helps him/her a lot in copying the actions of people or animals or vehicles something he/she likes to draw mostly. Many animators and artists even publish their books and most of them contains some pages represented. I do have a sketch book but it doesn't look like a book. I scribble a lot in separate pages then and there,so mostly we can call it a collection of pages rather than a book.
The best practise is to have a small plain paged book which will be either spiral or bounded that can be easily carried in a bag or in hand. It helped me a lot, i used to take it wherever i go mostly a vacation or distant areas. I try to copy the situations around me in quick drawings (mostly called 1 minute sketches).
It is a good practices to have one. By learning the actions of people around we can easily get ideas in animating a character. To start with there is no need to hurry and fill the pages. Once you are in a mood to enjoy drawing ( you should be if u are passionate), start drawing the things you see around. May be a small tree, children playing a cup , table peoples actions etc. It should not be a proper cleaned sketch. A rough sketch will do and has its own beauty. So start scribbling..