"De paso" is commonly used when something small is being done in addition to something bigger being done. (Since the "biggest" thing being done, takes away the problems or challenges of doing the "small" action.) For instance if you are running out of milk, you would probably wait until you need more items, or wait until you need to get on the car for a reason; before going out exclusively to get milk. Say if you will go to your parent's house to visit them, you might just as well stop at wal-mart and buy some milk, which you could say "Voy a la casa de mis papás, y de paso voy a llegar a wal-mart a comprar leche". Hope this helps!

It's like I write to you and also send to you... it's like also and could be apart too