TLDR: we often rush to building an initial version of a product when we can jump straight to manually creating value for a customer and testing demand by charging for it, then crystalize that learning into a product later.

I believe there's a corollary to this: you can jump straight to building an MVP when you're building for yourself and are confident there are others like you. I remember PG describing it (in a video that I can't find the link to) as a design pattern where the broadcast signal and receiver all within the same brain.

Even better is when the thing you build solves a real problem at a business you're running and it makes you more money. My favorite YC example is Ilya from Mixrank. He was working as an SEO consultant when he realized some of his process was abstractable as scripts, wrote some to help him do his job better, then emailed it around to other SEO colleagues to see if it helped them as well, and all of that validated learning helped him attract his technical cofounder and then build Mixrank as a product. I believe they may have even been profitable before starting YC?