He wanted the flight crash to make it look like the death of Dr Pavel, so he took the blood from Pavel and put it in other guy so that when they find the bodies after the crash they can confirm the death of pavel by his blood signature.
I think Bane wanted Pavel to be assumed dead because a nuclear scientist missing would bring needless attention to the plan he was forging to destroy Gotham, and I think sooner or later his connection to the scientist could have been exposed if he wasn't assumed to be dead.
There is one specific reference in the movie when Bruce Wayne talks about his nuclear energy project (when he is about to go bankrupt so he tries to market the idea to Miranda to back him) and during the discussion it was mentioned that Pavel was the only guy who could've helped him out with the success of that operation and when Bruce asks about his whereabouts it was revealed to him that he was killed in an air-crash (6 months prior or something, forgot the duration).
Hope it clarifies your question.