Having played various real time strategy games that have this (Battle for Middle-earth, Company of Heroes), it is probably the last important unit selection hotkey that is still missing from aoe2de,
the Select all units currently on screen.
An example:
As a pocket player, you may want to send two separate armies to two different places of the map, like the two flank teammates in a 3v3. The most efficient way to constantly push these two separate armies towards their respective target, is to use that new hotkey. It’s not that efficient to make (and update) ctrl-groups for two separate armies of mixed composition.