Cross-platform app development tools are programs that let you build apps for more than one platform, like apps for Android and iOS, using the same code base. The reason cross-platform mobile development tools are so handy is because there are so many different types of devices out there. If you want to release your app on as many app stores as possible so that lots of phones and tablets can use it, you'll need the app to support multiple platforms.
In other words, you will miss out on potential users if your app doesn't run on their devices. A cross-platform app builder can save you from needing to program the same app in different languages and in different mobile app making programs.