Using the Alt key while drawing or selecting objects is useful in the following situations:
Select an object behind an object already selected
To select an object behind an already selected object, keep the Alt or Alt Gr key pressed and click on the already selected object.
Select next object for editing
Select a line object and choose e.g. the Cut function. If you press the Alt key, the cursor changes to the Select Object and Edit Vertex mode. Keep the Alt key pressed and click the next object you want to cut. Release the Alt key and continue with the cutting.
Draw horizontal lines
To draw horizontal or vertical lines, press and hold the Alt button. The line snaps in a vertical or horizontal direction.