This kind of thing is irrelevant today because most social media proxy uploads any external images that you embed. It’s about security, that.
However, if you are still on a 2004 BB forum that allows embedding of external images, you might find that you cannot embed all images that you find on Google. Some of those images show a hotlink protection. It’s best to test first.
This uses CURL to test hotlink, so you will not make the mistake of seeing your own cache, making you think that you can embed it, but later you realize it’s a no hot-linking image, etc.
This old thing tests all that for you.