I work with Spring and Java EE professionally, prefer Spring wherever I can. Also use Spring for private/side projects (if not plain Java). But I'll talk about Spring here.

I see two import aspects that Spring provide: first, dependency injection/inversion of control as basis and then a way to 1000+1 things "right".

Dependency injection is important because it provides a healthy way to compose an application from many components or services. It's not exclusive to Spring of course, you can do DI/IoC without Spring (even without any framework), but it's baked into the very nature of Spring.

Next, Spring provides "right" way to do a lot of more-or-less standard things. You have solutions for ORM, REST interfaces, security, batch processing and a thousand more tasks. From my experience, Spring solutions are mostly good or very good, at least much better than I'd design in a limited time frame. This gets me faster to the working solution.

I also don't thing that Spring brings that much of a complexity overhead. Once you got past the first threshold (like, you can assemble you application from a few components) you're good to go. Ok, you will eventually need to learn concepts of individual Spring components (like ORM or security), but you'd need to learn them anyway.

Eventually you'll need to fight Spring here and there, I won't deny it. This is normally related to the "automagical" stuff like autowiring or autoconfigurations - things which make your work easier in 98% but might make you crazy for the rest 2%.