What the others have said.
When you have an original made from various materials (card, resin, bondo, plastic details, metal details etc.) this brings with it a number of problems: the item will be heavy, some of the materials may not take to paint as good as others, the areas where different parts meet might be susceptible to parting or breaking, or the bondo may crack after some use. The wall thickness may also be thicker than necessary.
With a cast, you have a thin, lightweight and durable piece. And because the whole thing is now a single part, it is far more durable, and even slight flexing won't cause areas to crack or separate.
I've never done it, but if I had to guess: the possibility to make more. If you cast an item for yourself, but you have fairly average bodily dimensions for said item, and said item gains a significant amount of admiration in the community, you could probably make a few easy bucks by doing a run of hat item (once you've decided you are content with not being the "unique snowflake" so to speak).
Besides for duplication purposes, a builder might make a one off cast because their original isn't made of a durable or functioning material. For example, the originals to many helmet builds are not hollow on the inside. By molding and casting it, you can create a light weight/thin yet durable shell, hollowed out for your head.
Or, and often the case, the original is made out of clay or wall plaster, mashed potatoes; some material that would not hold up well over time.
Another point to consider is weight. If you're making a full pep/resin/filler Iron Man suit, it's going to be pretty heavy. If you mold and cast the parts in a different medium, you cut down on the weight significantly
Another thought is, can you imagine spending all of those hours making something and it being destroyed by accident. A lot faster to pull something than to re-create it from scratch. Several hours saved to keep the set of armor in tact.
This is the main reason I am planning on casting most of my armor. (Still WIP) but when all said and done the originals will be on display in the house and I will wear a pulled set out and about
For me personally I created my own helmet because i have a big friggin head 24 3/8" isn't a standard size so if I want something to fit right I have to customize it myself.