Marshall is not a #1 receiver; he is, however, a big target and a possession receiver, a perfect fit alongside Beckham as the big play threat. None of their other receivers could fill the "move the chains" role.
Usually teams don't actually find success in the season to match their FA spending, but the Giants had success with the defensive moves last season, and this is a great move for the offense. Now, if they get Peterson or draft a RB, they will just need to find an OL to help them...because that OL still stinks.