Eight years ago the Rams picked up RB Steven Jackson in the first round of the draft and I'm not even sure they could've predicted how much production they would get out of him. He's carried the ball at least 237 times every season dating back to 2005, his second year in the league.
Richardson is the same type of back, but even more dynamic. This is a perfect fit. SB Nation's, Rams blog, Turf Show Times, explains the pick:
The Rams desperately need skill players on offense. Blackmon is a better fit given the need at receiver, but Richardson is a superior talent. He compares well with Steven Jackson, who has since lost his speed and explosiveness as a result of being leaned on so heavily in the last five years. Richardson can be a feature back or a one-two punch with Steven Jackson. The offense can use the pairing to create mismatches and draw the attention of defenses, which would ideally free up some space on the field for their receivers.
Trent Richardson 2012 NFL Draft Profile (via sbnnfl)