Riastrad after we get summoned by the Exarch we automatically attuning to an atheryte crystal (he says to literally attune to it as soon as we get inside the Chrystarium and that's literally the first place we go to in there) and it is assumed that we can teleport there like we would on the source. When we are first summoned there we are magically teleported by the Exarch because the same thing that happens to us had happened to the Scions. The beacon was placed there as a beacon to Guide him closer to our Aether and our Soul and give him something something more physical from his time and place to pull us Aetherically, and physically to him. With out that beacon/device he would have just kept pulling the souls of our Friends accidentally.