It pays to be aware of the four pieces of armor in Minecraft- for a number of reasons. First of all, you want to make sure you’re using your resources to the best of your ability. If you don’t have enough iron for a full suit of armor, which pieces should you be leaving as leather to save material without sacrificing too much protection? If you really need some heavily-enchanted armor and so can’t go full-diamond, which piece or pieces should you be replacing, and with what? This is where a careful eye to your armor bar is key.
First of all, the biggest piece- the breastplate. Offering the most protection per unit of material used, the breastplate is also the armor piece with the widest range of values- a leather breastplate gives you one and a half shirts, to the full four of a diamond breastplate. If your armor is piecemeal, this is the part you want to have built of the strongest material available, as it will be providing nearly half of the protection you get.
Second, you have the leggings. Leather pants only give you one shirt of armor, but diamond leggings will grant you thrice that. The biggest jump in difference, though, is the one and a half shirts that gold leggings give you upgrading to the two and a half of iron armor. Overall the second most efficient armor piece, leggings are also the other piece which has different armor values for each material.
Third in efficiency are helmets. While a leather helmet offers only half a shirt of armor, and the diamond offers a shirt and a half, both the gold and iron helmets grant a full shirt worth of armor. This makes the gold helmet the closest piece of gold armor in value to its diamond counterpart, losing you only half a shirt. If you need an enchanted piece of armor badly to go with your diamond outfit, this is the piece to make out of gold instead- you’re losing the least ground. Similarly, if you need to make something out of gold or iron instead of diamonds for the sake of saving materials, this is the piece you can patch over most easily with the weaker material. If you must make something leather instead of any other material, this is one of the two pieces you’re safest doing that with as well.
Finally you have boots. Leather and gold boots offer a measly half-shirt of protection, iron a full shirt, and diamond a shirt and a half. They are the only piece of armor where gold and leather are effectively interchangable, and if you have to use leather, use that here too. If you need enchantments badly, this should be your second piece of gold armor, being a considerably worse place to wear gold than on the helmet (though noticeably better than using gold for a breastplate or leggings).