Series D and beyond are typically used to extend runway to IPO, fund acquisitions, or finance growth that the public markets will price more efficiently than later. They're often led by crossover funds, sovereign wealth funds, or strategic investors. Round sizes range from $100M to several hundred million.
Valuations at this stage become increasingly sensitive to public-market comps. A Series D priced ahead of the comp cycle can become a 'down round' if public multiples compress before IPO.