Upon opening a project UE4 will generate various files and folders if they do not already exist:
Plugins/*/Intermediate(any Intermediate folder inside a plugin)
ProjectName.sln(only if your project has a c++ code in it)
These files and folders can be deleted to reduce the total size of your project when archiving, sharing and/or moving the project. They are often times also excluded from source control using a
.ignore list for example.
Binaries folder inside of plugins should not be deleted as they might contain pre-built libraries of which the source code is not distributed with the plugin and therefore cannot be regenerated on demand.