Engineers can move/remove parts in flight or on the ground, if you don't have enough dV you might be able to remove some parts you dont need anymore lower your weight. If you have more kerbals standing around they can help the engineer move heavier parts. This has saved so many missions for me.

I'll go first for an example. Docking two spacecraft together is a lot easier if you place the docking ports in normal/ anti-normal orientation first.