Most service members hear "GI Bill" and assume it's one benefit. It isn't. The Post-9/11 GI Bill and the Montgomery GI Bill are separate programs with different eligibility rules, different payment structures, and different tradeoffs if you switch branches, get out early, or want to transfer benefits to a dependent. Picking wrong, or picking without understanding what you're locking in, can cost you money and flexibility later.
Yellow Ribbon adds another layer on top of that. It only matters if your school's tuition and fees exceed what the Post-9/11 GI Bill pays on its own, and only some schools participate. None of this is intuitive from the outside, so it's worth walking through before you commit to a program.