Leather is a skin, with fibers and pores, that requires proper natural lubrication and needs to breathe. Because it no longer has a body to provide proper natural oils, we must provide these oils for it. The key to extending the useful life of your leather is restoring it with proper oils. Be warned, not all "natural" oils are proper for leather care. Unfortunately, many products claiming to be conditioners or preservatives are not. They contain harmful chemicals, mink oil, petroleum, pine tar, or alcohol to soften or waterproof leather. They soften by weakening or decomposing the fibers and they waterproof by sealing the pores! They're low priced but definitely not economical when you consider the damage done to fibers, stitching, and glues. Ironically, these "preservatives" actually shorten the useful life of your leather!
True preservatives are natural and may cost a little bit more, but they are your best protection. They last longer and do so much more to preserve and protect that they're actually the most economical way to treat your leather.