Being lost offscreen with no idea where you are is quite a nuisance in a local multiplayer game. To prevent that, SE has offscreen markers, so your comrades will know where exactly they are. The same markers will also be used in online multiplayer.
A small notice: Gamescom preparations are taking up a lot of free time, which is already pretty sparse. Thus, SE marathon updates won’t be made on a consistent pace - this post might even be the last of the current run. No reason to fret for the future of Mari0, though! Good things come to those who wait.