It’s mainly a user interface design choice.
Many graphics settings affect or interact with each other. When adjusting settings, you may need to change several and then apply them all at once to see what the overall effect looks like.
If each change was applied independently the instant you made the change, then you wouldn’t be able to easily see the difference that your overall selection of changes made.
Applying the settings changes all at once makes it really easy to do an A-B comparison between old and new settings.
Another reason for the “Apply Changes” design choice is that it allows you to easily cancel. The pending changes are staged, and you can easily back out of them without having to reload anything. If the changes were applied the instant you made them, then they would have to be undone in order to “cancel” them, and you’d end up reloading twice.