In some 5 back formations the LB and RB are actually LWB and RWB.. They are supposedly faster and are used at both ends of the field depending on the managers tactics. They are generally much faster but smaller than the CB so they are kinda a flex attack and defense option on the side. I personally prefer 3 CB and more MF with 1 or 2 ST (Fifa 12/14). However, ive started to try and make formations based on personnel available (when you start with teams that have a transfer budget of 10m, its difficult to do what ever you want), i find a 4 or 5 back system can be interesting and provide mutiple options for attack. In Fifa 14 with QPR, i used Luke Shaw as my LB, and it was a ton of fun creating passin triangles and thru balls with Shaw and then see him sprint up the field and cross the ball for a scoring chance.
Anyway, its nice seeing Rodwell get first team minutes in the premier league and even get a goal. There was too much talent ahead of him on ManC for him to have a legitimate chance at becoming a mainstay. I hope he does well with Sunderland