Meaningful statements are those that are either true by definition or are verifiable. A statement true by definition is a tautology ("tauto" meaning "identical" and "logos" meaning "word, idea"). An example of a tautological statement is, "A triangle has three sides." The other types of meaningful statements are verifiable. Verifiable statements are synthetic: they can be proven or disproven. Proveable or disproveable statements are testable.

This philosophical idea is known as the principle of verifiability.

Think of anything you believe to be true. An airplane can fly - true by definition. Improved aerodynamics reduce drag - a synthetic statement that can be proven or disproven. G*d exists - not true by definition and not proveable or disproveable. Therefore, the statement is not meaningful.

Making decisions based on meaningful statements is useful.

This is a synthetic statement: not true by definition. Thus, you should be asking: why is it useful? Usefulness to a human being means improved functionability. Making decisions based on meaning improves our ability to function in the world. The utility of meaningful statements is proved in millions of ways. You must eat to survive. Food must be produced or procured. Creation requires work.

Armed with an understanding of meaningful statements, I ask you: what commonly held theories are meaningful? Meaningless? If testable propositions about an idea cannot be made, can the idea have meaning? If a statement is meaningless, does it have value? You no doubt already apply the principle of verifiability to almost everything you do. But is there anything you don't use it for? Why?