A wire, is a conductor of electricity. No material known to man is a perfect conductor, so we tested everything we could find on the planet. As man evolved, we not only learned how to test for conductance, but also learned how to determine the opposite, resistance. Metals, the materials known to have the least amount of resistance, conduct electricity best. Simply put, the more metal you use, the less resistance in the conductor. Since size, cost, and space are considerations, folks tend to choose smaller wires if they don't fully understand Resistance. This page is designed to help you get started on reduction of resistance, but the choices are yours to make.
Why it matters? If you read the battery efficiency discussion here, you already understand that a battery isn't perfectly efficient either. When you add the efficiency of systems in a circuit together, you multiply each part's efficiency to get system efficiency, rather than add them up. When you do so, you find that little losses add up to big problems. For instance, this equation shows that 2, 80 amp flooded batteries, that are 65% efficient, with 85% efficient wires, only delivers 55% of the power stored in the battery to the stereo, trolling motor, or other electrical loads.
.85 * .65 = .55 for total efficiency of the system.
Then you multiply for the equipment size.
55% x 80 amp hr batteries x 2 = 88.00 ah deliverable.
To change the game, buy a pair of good batteries AND good wires to have more than double the power.
Better Batteries have more power in the same space (1.2-1.5x), and less resistance too, at 87% efficient.
Better wires bring an 85% efficient system to 95% efficient.
.87 x .95 = .83 efficient system.
83% x 100 ah x 2 = 165.3 ah deliverable.
When you want the best marine batteries, the wires also play a role.