Originally Posted by R_Hythlodeus
It doesn't, but it advances the character. It is called character progression. We know Selvig from 2 movies now, in the last one he was taken over by Loki against his will. Now we learn of the outcome of this, just like Tony's PTSD, Selvig is not untouched by the events of TA. He had a god in his head and has to deal with it. You can't do that with Thor really, showing how big an event TA for the characters was, because out of all Avengers, the alien invasion wasn't a big game changer for him. For Selvig it was. Thor has to deal with his brother and what Loki did, but for Selvig it was a bit more. Being possesed by a god should be adressed.
Selvig has no use in this film other than some scientific McGuffin at the end.