What do they actually do? I understand the purported significance of ERC20, but it is not clear why these tokens are actually useful. Literally anyone can mint them... in fact that's the point.

Sure, the idea is that now it's easy to mint your own currency to do stuff with. That wasn't so easy before! So Kik is minting a currency to serve as Weird Premium Kik Points, and GameCredits is minting a currency that you get by playing Candy Crush or whatever, and BAT is minting a currency that represents attention you paid to ads, and Augur is minting a currency that makes an incentive structure for reporting prediction market results, and so on.

This is plausibly better than if they had just rolled it by hand outside of Ethereum for a few reasons, e.g.

- You automatically have a way for users to trade real value for your currency via ETH. (This is probably easier for devs than setting up Paypal or Stripe integration, but harder today for users.)

- The currencies are something you have that's more tangible and auditable than an entry in some company's bespoke centralized database. Everyone can see it and believe in it.

- Users can easily send the currency to each other by default without all the transactions having to go through you. (I don't actually know whether this is a pro or a con for many use cases.)

- It's trivial to make interoperable software that accepts or holds or sends these currencies, like exchanges and wallets.

- The above facts probably mean that there is something like an open liquid market for your currency, if anyone cares about it. That means people can perform mutually beneficial trades! Mutually beneficial trades are mutually beneficial.

As to whether it's useful to have a bunch of currencies for different things, I think so? I mean, there already are a ton of things like this in software which behave as de facto currencies -- free-to-play games and MMOs are a good example -- and those seem like they would be well-suited to be ERC20-style tokens. And as someone who believes in incentive structures, I'm excited about the prospect of quantifying more things which are currently vagaries ("attention", "reputation", "compute time") into currencies with a market. Augur's design is a good example of this.