Uninstalling apps on the iPhone deletes the app and all the data related to it, from your iPhone device. So, at a later date, if you want to use the app again, you will have to re-install the app and then log in using the appropriate credentials. Some apps tend to restore the data once you sign in, while some of them just offer you a fresh start. It is highly advisable to think through these things before deciding on whether or not you want to delete the app from your iPhone.
