I'll give some my own answers. The reason I need estimates from engineers is so we can do planning. It seems simple. But engineers sometimes don't often have an understanding of where their work is situated amongst other business activities.
In the case of B2B SaaS products, when engineering is close to shipping something new, other departments gear up to receive the handoff and make it turn into revenue. That usually means marketing pushes, sales calls, customer success follow-ups, etc.