Preserving Value Across HMR With Global Object

When looking at the NextJS MongoDB Starter, I noticed this bit of code:

if (process.env.NODE_ENV === 'development') {
  // In development mode, use a global variable so that the value
  // is preserved across module reloads caused by HMR (Hot Module Replacement).
  global._mongoClientPromise = this.clientPromise;
}

Here is a link to the file I am talking about.

Now, I know of the global object, but what I do not understand is how merely assigning a value to it preserves it, since global._mongoClientPromise is never accessed thereafter. In fact, this is the only line in the entire project where this property is used. Maybe it is obvious to some, but I am not a JavaScript expert, so can somebody please explain this to me?