Saying that you "might as well" do something means that:

• you think it's a good idea to do it
• it's easy to do

For example, buying a lot of something while it's on sale is a good idea because it's cheaper. If the item on sale is also easy to carry and to store, then there's no reason not to buy a lot of it:

We might as well buy a few more.

Another example of "I might as well" is:

I guess I might as well stick around and have one more.