Most times you won't need a well-formed (REST) API to start with, and many times you will never need it. Most small projects should start without one and create one when need arises.
A rest endpoint is likely handy to use across your application without having to replicate the same serialization over and over again.
Also many frameworks remove all the boilerplate for those operations (eg. Django class views), so creating one is a really good starting point.