If you are developing your own XRP tool (eg. Social Multi Signing tool, Escrow tool, trading platform) or working in a e-commerce/retail/POS environment, XUMM will make the workflow of getting a transaction signed a whole lot easier.
XRP transactions are usually user initiated: open your wallet, enter the destination, amount, etc. and then you submit your transaction. In retail / e-commerce (and many other) scenario's, by "reversing" this process, the payment flow will become less prone to mistakes and much more user friendly.
This is where the xumm platform comes in. An XRPL transaction "template" can be posted to the xumm API. Your sign request will be stored (called a payload), so the xumm app user can open the sign request (by scanning a QR, deeplink or push notification) and resolve (reject/sign) on their own device.
When a user signs your sign request, he or she can trust your application. In this case an application specific user token will be generated for your application, and future payloads will be delivered straight to the end user with a push notification.