What's the purpose of xml files next to the dll?
They contain the comments for the code in xml format. They can be used with sandcastle to generate windows help files or MSDN style html documents.
This file contains description of the classes, methods, etc created as xml comments in the assembly.
The Microsoft Docs says:
The compiler generated XML file can be distributed alongside your .NET assembly so that Visual Studio and other IDEs can use IntelliSense to show quick information about types or members. Additionally, the XML file can be run through tools like DocFX and Sandcastle to generate API reference websites.