That is the precompiled headers file that is used by the IntelliSense parser. Distinct from the precompiled header file that the compiler generates when you build your project, that’s a .pch file in your build output directory. It otherwise plays the exact same role, it greatly increases the speed of the parser. It can use that kind of help, the EDG front-end has never been particularly fast.
Deleting them is fine, they are only used when you have the project loaded in the IDE. If you reload the project then IntelliSense is going to be catatonic for a while, rebuilding the .ipch file, re-parsing the files in the project and recreating the .sdf file in the project directory. On large projects that can easily take a handful of minutes. Of course, the bigger the .ipch file, the longer that will take. It is something you’d normally only contemplate after you finished a project.