Leather is tough but we must also remember that leather is in fact skin. It can dry, crack, stain and warp. Their are various options for treating and taking care of leather so that whatever form it takes can be passed down your family line.
Polish is largely actually for aesthetics. On its own, it’s mostly about the actual shine of the product (usually shoes and bags) rather than protecting it from the elements.
Conditioner / Cream
Leather conditioner or cream moisturizes the material so that it doesn’t dry out and crack. This won’t make your bag “shine,” but it will protect them. Note that in most cases conditioners don’t waterproof your leather (though some do include a water-repelling component). Leather conditioners are lotion-like, and are to be gently rubbed into the leather. The hide will soak up the conditioner, just like your skin would soak up a moisturizer.
Using a damp cloth is the old reliable when it comes to leather care. Since leather is so very naturally durable anyway, giving it a wipe down (without soap — its chemicals can ruin the leather over time) once a week to get rid of the dirt and dust constitutes a minimally sufficient care routine. If you use nothing else, this is the way to go.
Leather needs to breathe. Just like skin, leather needs some ventilation to prevent mildew and rot. Air can naturally pass through leather, leaving moisture to evaporate naturally. That can’t happen when your leather is all sealed up, though. So don’t ever store or transport it in a plastic grocery bag. Either use the storage / travel bag the item came with, or some type of breathable fabric — pillowcases are great for shoes, bags, and / or other accessories.
Keep leather away from direct sunlight / heat. If a leather item gets waterlogged, it can be tempting to throw it in front of a heater or to use a hair dryer to speed the process. Don’t do that, ever. Just like skin and other fabrics, when leather gets wet and then heated right away, it can shrink and dry out too quickly. Rather, let it dry naturally, even if it takes a couple days.