The reasons they give are very weak.

It gives us control over our own destiny.

This is the only reason that makes any sense: they wanted their own coin so they can get a head start in minting their own money, in the way that the federal reserve has control. I'll accept that. Everybody wants to be rich.

It helps align incentives across our decentralized network. Miners get rewarded in Siacoin for securing the blockchain, hosts receive Siacoin for selling storage space, renters pay in Siacoin to buy storage space.

This is a benefit of blockchain but not of creating their own coin. Nothing I see here that you can do with sia that you can't just as well do with any other coin.

It allows us to create smart contracts built specifically for file storage. A Siacoin can do 2 things - it can be transferred from one user to another, or put into a file storage contract on the network. last thing we want is to run an enterprise-grade storage network on a technology that we do not control. If Bitcoin or Ethereum have network congestion, or if Ethereum forks off, or if some new vulnerability is discovered, we don't want to be affected, or subjugated to another party's roadmap.

This does nothing to further the argument, they're just more justifications of reason #1 stated in a circular fashion: "we created our own coin because we wanted to create our own coin."

Ultimately there's no benefit to the consumer, which would be fine were it not for the additional overhead of tracking yet another crypto. There a reason why we have a currency. If every business insisted on all their customers paying in that businesses' "credits" instead of cash, it would be an absolute pain for consumers to have to keep track of all that.

OP's point I imagine is, it's OK for you to ask me to trade in your special "credits" to do business with you, but you need to bring something extra to the table if you're going to ask me to do this additional bookkeeping.