When in object mode, a special actions window titled obj will appear, which allows you to change what the left mouse button does. It can be hidden or unhidden with right mouse button.
Add either creates a new object or moves an existing object. Shift clicking an object will select that object. The delete, raise and lower actions will do just that. Raising and lowering objects is useful for concealing weapons behind crates.
When toggled, the snap button will snap newly placed and moved objects to a 16x16 grid. There is no way of changing the grid size.