I'm using Spring Tool Suite and learning Spring. I've created a Spring Start Project. The project wizard ask for the version of Boot.
I'm just practicing Spring and doing output to the console. Why do I need Spring Boot? There's not way to create a simple Spring project without it.
Nobody forces you to use Spring Boot. It is probably the easiest and clearly the recommended way of using Spring nowadays, but nobody forces you.
Since it is the recommended way, we provide support for creating Spring projects using Spring Boot in STS and help you with that a lot, using the initializr in the background.
But if you want to use just the Spring core framework, you can create a Java Project yourself, choose a dependency management mechanism like Maven, and add the dependencies to the core Spring framework yourself. There is nothing that prevents you from doing that - and there are no dependencies from the core Spring framework that require Spring Boot.