As smartphones became popular, came the advent of mobile apps which have taken the world by storm. There are mobile apps in various categories now ranging from text messaging apps, navigation, weather forecast apps, social media apps etc. You think of anything and you are likely to find an app for it already present.
Text messaging apps have recently become very popular because it allows people to send unlimited number of messages to each other completely free. Your friend may be in another country; they may be using any service provider and may have different kinds of mobile phones. You can still text them any time of the day or night with just a couple of clicks on your smartphone.