The Rams need help at a lot of positions and Worthy is one of the best players remaining. Defensive tackle is a tough position to fill, so if the Rams can find that guy in the second round, all the better.
SB Nation's Rams blog, Turf Show Times, explains the pick:
Gregg Williams is exiled, and nobody has a good idea of what exactly the Rams' defense will look like in terms of scheme specifics. They signed Kendall Langford, and Worthy will be a good pairing in the middle of the front four. Some say Worthy will need pushing from his coaches to realize his full potential. The Rams finally have a coaching staff known for getting the most of their players.