Sin still ruins lives on Earth, which is why we shouldn't do it. But not only that, but there's also the baptism into Jesus. Water baptism involves submersion (or water-sprinkling), and there's a special significance that connects to baptism into Jesus. In fact, the process has some striking parallels.
First off, when we are baptized into Jesus, we "were baptized into His death" (6:3). Dying with Christ basically means that the old flesh lost its ability to remain dominant (and couldn't "live"). This also leads to getting "buried with Him".
And with dying in Christ comes rising with Christ. Similar to Christ's resurrection, we "also should walk in newness of life" (6:4). Jesus's resurrection is the renewal (and demonstration of his power over death), and we know that Jesus provided us His Spirit (which is basically new). This new way of living is designed to surpass the old way of living. So why rely on old styles (that ruin lives and lead to death) when we have a new style based on Jesus's power over death?