If your app is not app-to-date with Unicode standard, or simply uses font which not supports all of Unicode characters (and, frankly, there is no font that supports all of them!), you would get ��� in text.
In such cases, you can use this app to show you meaning of this missing characters.
Also, take emoji. Even then you see it as a small picture, maybe you don't see exact meaning of this. E.g. age, sex, skin tone, hair length, and many more attributes are defined in emoji, and you see (recognize) only some of them.
Or, maybe you can see tree and road, and you don't know what it is? Check it with my app, U+1F3DE character means "national park" (by the way, sometimes it is rendered not as tree, but as mountain).