When creating Facebook video and image ads, should I create regular posts or build them in the ads manager?

Should you always create video and image ads as regular posts on your page and then when you create your ad, just select them as an existing post or are there certain times when you don't need to do that? You can just create, upload an image and write the ad copy, right from the ads manager? And is there like a good rule of thumb to know, you know, when to do what? So the only time you want to do it right from the ads manager is always...