iMessage is supported by some satellite networks already, and the constrained networks manifest value would be useful to allow the app to work on satellite networks.

In the developer documentation, it also gives apps the ability to detect when a satellite network is being used. This can be used to show the user they are on a satellite network, and what to expect (similar to how Google Messages handles it). This may also be useful for preventing iCloud sync until on an appropriate network, preventing media from downloading automatically, and showing warnings if the user attempts to send an image or video.

(Some examples of these satellite networks are Verizon, T-Mobile, O2 UK, ROGERS)