Phil: You’ve been working for three hours. Come take a break with us.

Steve: Okay. I guess I could use a breather.

When we say we “could use” something, it means we would like it, although we may not actually really need it. After working for three hours, a break would be nice. Steve has been working hard and deserves a break. But if he really had to, he could keep working. (A “breather” is another word for a break.)