Sharepoint - What are the Type.registerNamespace advantages over standard JavaScript methods?

There aren't any technical advantages to using this method. It simply allowed developers to use the same conventions they used on the client (JavaScript) as they used on the server (ASP.NET SharePoint solutions).

The JavaScript Object Model code is transpiled from the C# Client Object Model -- lots of things in the Microsoft Ajax libraries exist to make this transpiling work properly and to make working with JavaScript similar to working with the C# libraries, which made client side coding easier for ASP.NET developers to get started with.

This would have been advantageous at the time the JSOM was developed (circa 2009) since most SharePoint developers would have been experienced ASP.NET developers and not necessarily experienced JavaScript developers.