What are the benefits of establishing a clear Content Model before beginning your development process versus constructing one as you go?

With a clear content model at the beginning you normalize your content in a structured way. You will have a better understanding what pieces of content will be needed for the presentation layer and as meta content as well. All content fields should fit for its use if it is good planed. Otherwise when you adjust content models as you go you probably affect created content as well. For example when a field type has to be change you will have in worst case a data loss and/or some adjusments needed in design and development  too afterwards. 
For a good development and design the whole content type should be ready for use so they will build their work on a good fundament