Someone near and dear to me recently opened a business (let's call it Generic Company) that runs entirely offline and is pretty low-tech. So I told them it would be a good idea to buy the domain right away, even if they didn't plan to use it.
Second, these days very few companies are so lucky anymore to find the exact CompanyName.com domain available for purchase.
When you add a custom domain Vercel suggests serving the main domain on www.genericcompany.com and redirecting genericcompany.com requests to it.
I'm glad I had the foresight to check for my "client's" company domain availability.