Visual Assist and the built in Intellisense tooling share a few common features. Both give you IntelliSense and code navigation — the difference shows up in large or complex codebases. Visual Assist uses its own parser, which means it stays fast and accurate in projects where built-in IntelliSense struggles: Unreal Engine, macro-heavy code, partial builds, and anything past a few hundred thousand lines.
Also, the Visual Studio UI can arguably complicate some dialog boxes for simple tasks.
An example is the Visual Studio unified search, which operates like the search bar in the operating system wherein everything is shown. In contrast, VA uses different search dialogs for each type of search, each with features specific to that kind of search and without displaying results you aren’t interested in.