Casey Hampton is set to enter free agency and at 35, his career may be at its end. Even if the Steelers bring him back on a one-year deal, upgrading the middle of their defense is a must. Williams may be coming off a knee injury, but it shouldn't negatively impact his future play. He's a powerhouse nose that can push the pocket or take up multiple blockers.
Williams, an Australian, is still relatively new to football, so his potential for growth is higher than many other alternative selections. He didn't accumulate gaudy stats for Alabama, but did his job anchoring the middle of their defensive line and disrupting opponents' run games. With it looking likely that the Steelers will lose Casey Hampton, Williams could slot right into his position at nose tackle.
A pass rusher could be an option for the team at 17th overall, but most of the premier pass rushing options will likely be off the board by then.
For more information on Williams, check out his SB Nation scouting report. Last season, he had 37 tackles, 2.5 tackles for a loss, and one sack for the Crimson Tide.