If you are stretching limits of java, then you would need support because that knowledge base isn't available easily. I worked for a company that had performance engineers from sun on site helping us with performance issues.
Or because your company has a policy of not putting anything in production without a signed license and/or current support contract. (See e.g. SQLite which is public-domain but if you want a commercial license anyway they'll sell you one. They offer non-FOSS extensions now but they sold commercial licenses before that.)
They are now using openJDK and modifying the source code themselves, so no, that's not the policy.
Yup. My company has a license/support contract for Spring Boot (among many many other things).