Re: Pictures of bubble level ...
I use the same level. When shooting from a bench, I have little difficulty keeping things stright up and down. In the field, off a bipod, things are different. Changing locations, uneven ground, visual obstacles, and prairie dog fever all add up to canting your rig. The level is a nice tool that helps you 'learn' the proper position for you and your rifle. As time goes by and your experience increases, you'll find yourself less dependent upon the device.
Anything that boosts shooter confidence is generally a good thing.