I've had problems with disappearing/wandering companions, and I've seen posts of others who had the same problem.

With a Companion tab, we would be keep track of our companions (which would be increasingly important as new companions are added, whether through future DLCs or mods). Location could be shown on the map of where their current position is, and to flag them in case they are wandering in the settlements or away from them.

And being a tab, a player could keep 'tabs' on their wayward Companions regardless of which platform they use to play the game.

Having to go back to earlier saves may help you get your Companion back, but there may be a lot of content you've completed in the meantime that'll be lost.