Because when you return as “your” reader, you can spot a lot of things very easily. Not only typos or sentences that don’t work with the knowledge you have as the author of the story. But also when it comes to plot coherence, contradictions, character head-hopping, setups without payoffs (or the reverse situation), sections that drag or rushed scenes, and where attentions slows and boredom creeps in, the parts you skim, etc.