An important argument is that Theme Inheritance deduplicates functionality, rather than reinvent it. Which is to say, it is easier to add on or adjust the large feature-set most themes come with. You don’t need to update the parent-theme if you don’t want or need to, but having a child theme often makes it easier to ascertain what changes you made to the original without digging into code-history and comparing changes. You can just switch between them.
Some themes also optimize for this, allowing more advanced functionality to override the original theme, without the need for monkey-patching code.