SDL provides..

..an image decoding library.

..an audio input/output library.

..a multi-channel audio mixer.

..ttf and rtf font rendering.

..networking.

..runtime shader cross-compilation

..its own entire graphics abstraction layer / API.

..its own shader language, shader compiler, and shader bytecode format.

I don't know at which point something becomes 'high level', but SDL is only 'minimal' if you use a subset of its functionality.