If you ever tried to trace the contour of something with the Lasso Tool you might found that it was appropriate tool for organic things. However sometimes we need straight lines for the selection and that is where Polygonal Lasso Tool comes in place.
The thing is you can mix both in the same time.
If you press and hold the Alt (Option key on Mac ) and then release the (left) mouse button, you'll temporarily switch to the Polygonal Lasso tool so you can draw a straight line. If you hold the mouse button again can continue to use the Lasso Tool. When you release the Alt (Option) key and the mouse button, Photoshop completes your selection with a straight line.
How wonderful it is.