Potentially we get to see a world class player in the team this season if he knuckles down and gets on with it, maybe even beyond that.
There are other more intangible benefits though.
Players hopefully realise that signing a long contract means they're signing it to play for Saints.
Keeping a player like VvD, appearing to knock back a world record fee, and humiliating Liverpool in the process, it starts to change the perception of Saints.
Out of the teams above us, Liverpool are possibly one of the weakest with that defence, with the potential for a terrible season, especially with Europe to possibly bog them down. If we're going to to get higher in the league, avoiding strengthening them is a decent start.
For me the big one is that teams might think twice before tapping our players up. The damage might have already been done with VvD himself, but if doing this means teams are reluctant to get burned again, that we know about interest in our players as it happens, and it's more difficult for them and the players themselves to go behind our back... that could be a huge benefit for us going forward.