When you're small, you usually don't want to commit to e.g. 3-year cloud spend. What if your business doesn't take off? Early on, long-term commits are a liability.
At certain scale you're inevitably going a fixed subscription (committed use) in order to get volume discounts. Depending on how high in 7(8)-figures spend territory you are per annum, these NDA'd discounts will be larger than many think.
At the same scale you also use commits to protect your business from stockouts during events like holiday season. You get a TAM (or a team of TAMs) that will help you bypass some standard support procedures and escalate your requests fast. You work with them on capacity planning for the next 1-2 years, etc.