[@TheNoCoKid]
To expand on this, maybe a summon system? You can summon out a digimon from a farm for a single attack, then need it to recharge both the ability to summon and, for a longer period of time, that digimon in particular's ability to be summoned. DiM cards would then let you do this with digimon you don't actually have on an island and with more regularity, less time to recharge the ability itself and somewhat faster recharge for the DiM than for the digimon on the farm. Maybe a special combo attack if you have two compatible digimon on the farm or on the farm and a DiM card? Doubles or triples the summon recharge, but is exceptionally powerful. Maybe another benefit of the VB digivice is the ability to use a combo attack with two digimon on the farm AND a DiM card, for an extreme attack once a day or less, giving tons of variety to manage against the combo attack of a SP that boosts the combo attack's damage anyway. 

Though saving the previous stages on the digivice seems counter-intuitive in terms of an actual game based on digimon that's similar to Pokemon GO, since it disincentivizes players to get multiple of the same digimon, thus spending more time and/or money on the game.  

[@Big McLargehuge]
I have another question that I forgot to ask: How much digimon media exists IC, and how does it differ from IRL digimon media? Obviously D:E is different, but do the anime and/or manga exist, or the video games, or the card games?