Storytelling is all about communication, right? You, the writer, have something to say. Ideally, you present it in a way that's authentic to you but also communicates to your readers exactly the meaning you're after.
If you "talk" your story too fast, readers can miss the point. Just like in real conversation, they may start to get confused or irritated, or disconnect entirely.
If you "talk" too slow, same problem. They'll skip sections. Ever do this yourself, when reading? You know what I mean.
So skilled writers (communicators on the page) find a "pace" that fits their stories. When the scene is tense, the pace speeds up. When we're absorbing meaning, it might slow down.