The answer to such a question is always: “It depends on what you need!”. So the actual question becomes: “Which conditions make Appium suitable for me?”. The most important assumption is that you are developing apps (pretty obvious I know). If you are developing an app for a specific platform (and have no intention of supporting others in the future), Appium is not really required and this is basically the answer you are looking for.
Appium becomes meaningful only when you have apps targeting more than one platform (Windows, Android or iOS to cite some). Appium becomes essential if you have a webview-based app (necessarily) targeting many platforms out there.