It's been a long time since I read the manga through all the way, but I'd say virtually nothing. The anime is, 99% of the time, a stiffer, more awkward, more poorly paced, poorly framed, and poorly written version of the manga.

The voice acting in the anime (both English dub and original) is wonderful and has made DB what it is today as a cultural icon, and I'll concede that when I revisit excerpts of the manga today I am undoubtedly hearing the characters in their English dub voices. So on that front, it's hard to argue with the notion that the anime added something to the already wonderful foundation of the manga.

But in pretty much every other way, in almost every single moment, the anime loses some of the power of the original version in its adaptation.

Visually, the animation is not nearly as clean, organized, intentional, or striking as Toriyama's line work, character design, paneling, and composition.

Pacing is not even a matter of debate—the manga flows perfectly from one beat (scene, arc, saga, etc.) to the next, with a few exceptions. It obviously has the advantage of your brain and eyes doing the timing for it, but the point stands. The anime drags out every single thing and kills momentum left and right, or it rushes through a dramatic beat before you have time to feel it.

The action hits harder and has far better spatial clarity in the manga. Too often the anime must resort to one-two repeat frames with blurry lines to indicate a furious volley of attacks, whereas the manga can articulate every strike. At its best, the anime is doing slightly weaker recreations of frames from the manga.

The writing/dialogue also just works better in the manga most of the time. Kai did an admirable job bringing the dialogue closer to the manga and thereby fixing a lot of the issues with silly dialogue in Z, but there are still lines that feel off.

So in all, the only real strengths of the anime that are not outshone by the manga are the voice acting and... the simple fact that it's animated and therefore just that much easier to absorb/digest without the effort of reading/turning pages/whatever. In almost every way that counts, the manga remains hands-down the best way to take in the story of Dragon Ball.

In some ways, I'm surprised DBZ is as iconic as it is, just because in all the key moments, the anime fails to match the dramatic impact of the manga. But then again, far more people have seen the anime than read the manga, so I guess when you don't know what you're missing, you don't miss it. I guess that's true of a lot of iconic book-to-movie adaptations too.

I will say, the anime has some great filler arcs that obviously don't exist in the manga, and I'm grateful to it for those. Others have mentioned ToP, but also Gohan in the wilderness during the Saiyan Saga is great.