In Android applications, often the choice of selecting an app to open a link or do some other action is left to the user, i.e. the framework lets the user choose the app to do something.
For example, say you have a link to a tweet, and you are allowed to choose between:
What is the reason that the user is allowed to choose the application with which to open a link rather than with native application?
According to this:
An implicit intent specifies an action that can invoke any app on the device able to perform the action. Using an implicit intent is useful when your app cannot perform the action, but other apps probably can and you'd like the user to pick which app to use.
and this:
The real power of intents lies in the concept of implicit intents. An implicit intent simply describes the type of action to perform (and, optionally, the data upon which you’d like to perform the action) and allows the system to find a component on the device that can perform the action and start it. If there are multiple components that can perform the action described by the intent, then the user selects which one to use.
The idea is that in Android, a developer can let his app use another app on the device to perform some task, instead of having to recreate the same functionality within their own app. Often, there is more than one app that can perform the same task, and so Android allows the user to select which app they want to use for that task.
Basically, the framework attempts to provide the most general way of getting a task done: if the official Twitter app is not present, then you always have Chrome.
The way this is done in code is shown here.