YouCompleteMe takes a built-in autocomplete engine and pumps steroids into it. YouCompleteMe has a few distinctive features that elevate it beyond built-in autocomplete:

• Semantic (language-aware) autocomplete; YouCompleteMe understands your code a lot better than built-in autocomplete
• Intelligent suggestion ranking and filtering
• An ability to display documentation, rename variables, autoformat code, and fix certain types of errors (language dependent, see https://github.com/Valloric/YouCompleteMe#quick-feature-summary)