Budgeting and planning (winner: waterfall process)
With a waterfall process, we decide at the start exactly what we intend to accomplish. We can therefore scope and schedule the project. We can also determine staffing and costs. Budget decisions are easy – IRR and ROI can be calculated – we can calculate expected values for both costs and (forecasted) benefits. With an iterative process, we're saying "I reserve the right to change my mind later". We fully expect to change the scope of delivery mid-project. We know we will learn more about how to forecast the benefits after each incremental release. Because we know about, and even desire future changes, we can't reasonably estimate ROI. We can "fix" the budget (aka timebox the project), but we can't predict the value we will achive within the time allotted.