1. You don't need to wash it
Not everyone agrees with this statement, but I have not washed my cast iron pans in years. I don't even rinse them (*gasp*). My rules for care include cooking with lots of fat (good kind), leave on stove with whatever bits are left in the pan and then repeating. I do cook frequently so food is never sitting in there very long.
Non-stick is a relative term. No you cannot fry an egg in it and expect it not to stick, but it sure beats the crap out of stainless steel. If you have a new pan, it will take a little while to develop the non-stick surface. Always remember to use lots of fat and it will become better and better.
3. Retains heat
Cast-iron is great for retaining heat, which is essential for getting a great sear on meat. I cook a very mean steak and my secret is using a very hot cast-iron pan. My other secret is #8.
4. They last forever
Not literally forever, but at least your lifetime.
This is probably my favorite reason. You can buy a 12 inch pan for less than $30.
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(price as of Aug 7, 2015)
6. Don't need to worry about scratching it
I hate having a nice shiny new pan and I use it once and it gets a scratch. Well cast iron does not really scratch. Or at least it does not do any damage to the pan, so go ahead and use your metal utencils.
7. Good for you, adds iron
Along the same lines as #6, if you do manage to scratch off some of the surface of the pan (not easy to do), it does not contain harmful products like most Teflon-coated pans. Cast iron is actually good for you! Cast iron can actually leach iron into your food, which is a good thing! Iron deficiency is fairly common, especially in women. Cooking food in cast iron can increase the iron content of your food.
8. Can use in the oven
You can take your cast iron from the stove-top right into the oven without needing to transfer to an oven-safe dish. This is very helpful for cooking a perfect steak or pork chop.
9. Build arm muscles
In case you do not already know, cast iron pans are quite heavy. For some that is a negative, but for me it is an extra chance to build those arm muscles. Talk about multi-tasking!
10. Self-defense tool
Sure knives are also great kitchen self-defense tools, but so is a huge chunk of metal.