Patting the zucchini dry with paper towels helps to remove excess water. This is especially important in the summertime when garden zucchini is really bursting with water. In the winter zucchini tends to not be full of quite as much water and you can sometimes get away without patting the squash dry. Too much moisture can make the bread runny though, so I usually pat my zucchini dry to ensure my loaf isn't too wet.