Simply put, the team that wins the first leg will have a greater chance of advancing to the final, because psychologically they will enter the second leg more comfortably. Actually, the four teams currently in the semifinals have the capacity and deserve to win the trophy, they have fought hard to get to this point. What distinguishes them is their experience playing in European competitions, regarding their performance and consistency in domestic competitions I don't think it can be used as a complete benchmark.
Man United gets more attention because of their reputation as an elite Premier League team, and once triumphed in the Sir Alex Ferguson era. But we must not forget that, the Europa League is where unreasonable surprises often occur. Like Atalanta victory last season, many people did not believe Leverkusen lost in the final where they were highly favored. Bodo/Glimt is a team that is not really considered, it could be that this season they silence many predictions.
A victory in the first leg can determine a much bigger step to qualify for the next round, but Manchester United can play much safer by relying on a draw like when they faced Lyon in the previous round. That doesn't only happen in the Europa League because almost all competitions have similarities and Manchester United must really focus because they face quite tough opponents.
The chances for Bodo / Glimt to come out as champions are very small even though we can't ignore those who have so far managed to qualify for the semifinals. But luck may also have to be accompanied by the quality of the players they have and of course Atalanta is much stronger because they are in a more competitive League.