" why is og:description a thing when we have <meta name="description"> and why specify og:title when <title>"

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Because facebook/twitter/linkedin have different ways to display information and sometimes what's inside the description (for SEO purposes) is NOT the same as what you want displayed on a social media platform or other outlet for your content!

<title>, <meta name="description" ...> nearly always get used to optimise for Google and not the end user of a social media platform.