On Android, when using landscape mode, certain apps override some user settings and "force" the keyboard to go fullscreen (see first screenshot - WhatsApp app with an Android fullscreen keyboard). The input text-field takes up the rest of screen, so users can see the keyboard and the input text area only.
Compare that to the rest of screenshots where the input text-field is smaller and users can see other context.
The two versions are as follows:
a) WhatsApp fullscreen keyboard: Users can't see any context (i.e. previous messages). The input text-field is taking up half the screen. The keyboard app (buttons and text input box) take up the entire screen. This is a chatting app that users mostly use it to send short messages.
b) WhatsApp normal landscape keyboard: It allows users to see previous messages (see what they are replying to) as it uses the app's own text-input field. The buttons and text input box take up about 60% of the screen depending on the app. You are also able to see context in apps like SMS apps which will automatch contacts or Gmail which will automatch recipients.
1. Why would a messaging app such as WhatsApp want to do this by default?
2. Why would any app want to do this?
(I see you have a larger area to compose but is that actually useful? It's easier to remember what you've typed yourself. Wouldn't portrait be better? Or making it an option?)
For those saying this is standard Android behaviour, here are some screenshots proving otherwise. You can clearly see I am using the keyboard in landscape mode but the input text-field is not taking up half the screen as in a fullscreen keyboard!