Why Does This Website Has the isPartOf Property Under the mainEntityOfPage Property?

I am in the process of learning schema markup, and one of the Blog Posts I reviewed as a reference has the isPartOf property under the mainEntityOfPage property. The main Schema Markup type in this case is Article.

Can you help me understand what was the reasoning behind this decision?

Here's the code:

{ "@context": "https://schema.org", "@type": "Article", "headline": "Hello World", "name": "Hello World", "mainEntityOfPage": { "@type": "WebPage", "id": "https://example.com/hello-world#webpage", "url": "https://example.com/hello-world", "isPartOf": { "@type": "WebSite", "id": "https://example.com/#website", "url": "https://example.com", "name": "Example Website", "publishingPrinciples": "https://example.com/publishing-principles" } } }