The only reason I've ever thought of making an alt would be to sell/buy items specifically for the FC I am the leader of. For example, if we are spirit binding with the intention of selling the materia to make money for FC housing... I would sell it on my alt so none of the money from the sale would get mixed in with my own personal money. Keeping track of all of that would be a real hassle when I could just sell everything on one alt and have her dump every single last gil into the FC bank whenever a sale is made.