Student or Learner
I would like to know the difference "1" and "2".
1. While you are at the store, get some milk, too.
2. While you are at the store, you might as well get some milk, too.
That's right. The first one phrases the idea as an order, but the second way of expressing it puts it into a form of an objective suggestion.
"Pick up your toys!" compared to "C'mon, kids -- it's time to pick up all these toys" does the same thing (using a different syntax strategy.)