Just to expand on my answer from the other thread, a miner could spend his dust and nuggets at an establishment but it came at a much lower exchange than that of an assay office (about $8-10 paying by the pinch versus about $15-15.75 at an Assay office. Remember, shopkeepers and bartenders would take the nuggets and dust from their tills to the assay office so giving the same rate of an assay office was not profitable.
Your exchange was paid out either in coin or bullion from the assayer. Since gold is so heavy, established banking firms such as Thos. G. Wells, Adams Express, Henry M. Nagle, Burgoyne & Co., etc.were relatively safe and reliable to deposit with. If you wanted to deposit your gold, it was more safe than a Mexican Bank (burying it). If you wanted to take your funds back to the states (ex. transfer $1000 in specie back to Boston) You would go into the express office, deposit your gold and 3 checks would be generated. The first belonged to the deposit branch, The second would be your receipt and the 3rd would travel to your consignee branch in Boston. It was a lot more secure than carrying it all home yourself. Many first hand accounts that exist from the California Gold Rush testified to this. Once you arrived in the Boston branch, your receipt and penmanship would be verified with that of their copy and the gold was yours.
I am actually writing a book on the topic with hopes of it's completion by this time next year. Although I am glad to go over content for the benefit of the discussion, I would request that you not reproduce this content in your own work. I am including the following content with sources cited.
Thanks
-Dave