By resigning Tank Williams, the Vikings insured that they would have both depth and a player with the ability to level wide receivers starting at strong safety. Assuming that Dwight Smith is cut due to the depth at the position, his off field shenanigans and his lack of a strong backer in the coaching staff, the Vikings have set up a training camp battle between Greg Blue and Tank Williams. And both of them can absolutely destroy people, something Smith wasn't known for.
Blue demonstrated his ability at the position during his rookie year while filling in for Smith against the Redskins and Packers, and more importantly, showed that he has the ability to make a wide receiver remember him, a trait he also showed in college.
Even if Blue can't beat out Williams for the starting role, the Vikings will still have an enforcer back there. I don't have video evidence of this, but the man's name is Tank after all. You don't pickup a nickname like that by intercepting passes.
[On a draft related note, the Star Tribune has a nice piece today about why the Vikings should think twice about drafting a wide receiver with their first round pick that you should make sure you read]