answer:I think they allow you to send a text to someone without using up the number of texts allocated to you on your normal cell plan. If you’re restricted to, say, 100 texts a month but you regularly want to send more, you could use the app to do so. They would need either Wi-Fi or your normal mobile data plan to work. In the latter case they would use up a small amount of your monthly data allowance. Or you could use the app on a tablet that has wi-fi but doesn’t have its own 3G/4G mobile data or texting facility.