Christ will return personally and visibly so that all are aware that he is coming. This is the sign the disciples had asked about.
The words imply that the location of the mortal remains of the elect is important. Why is this, given that the souls of the elect are with Christ from the moment of death onwards? It is because the importance of the body is never minimised in the Scripture. Men care for the body after death in spite of the fact that it is buried in dishonour, and rightly so. How much more should it be honoured at the resurrection. Of course, mortal remains are in some cases consumed by fire, or devoured, or scattered far and wide, and bodily particles may have been reused in other bodies. But this is a supernatural retrieval based on the knowledge of God. A body is not identified by the precise collection of atoms that make it up.