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

Completely agree with you. Establishing a clear Content Model before beginning the development process offers several benefits compared to constructing one as you go. It ensures consistency in structuring and organizing content throughout the development process, saving time and effort that would otherwise be spent on restructuring and modifying the content structure during development. Additionally, it promotes reconciliation and collaboration between different stakeholders, including developers, content creators, and editors. Although adjustments and refinements to content models may still be made during the development process, having a clear initial framework provides a starting point and reduces the need for rework.