Why a statement of work is important
A statement of work is vital in any motion design project as it helps both parties see exactly what work will be done in the project. The statement of work removes any ambiguity about the work you'll be doing and what the client can expect in terms of deliverables throughout the whole process.
Ultimately, the statement of work is important because it helps protect against the dreaded scope-creep. With a statement of work you have a something in writing to refer back to further down the line, so if the client asks for something that wasn't included initially, you can politely point them to the document and say: "as per the statement of work this wasn't included in this project, however we can absolutely look at including it with a revised budget and timeframe."