TL;DR: To ensure that Beeminder stays around.
Here’s a blog post from 2011 which I still think is relevant today, that explains some of the reasons why you should contribute financially to keep the websites and services you use up and running, and what happens when you don’t.
Lool I pay them enough money as it is from derailments.
But yeah, if you never derail then paying for premium is a great way to support Beeminder 
I think I think of it as:
Me paying for derailments: Punishing myself for being too lazy.
Me paying for premium: Rewarding them for providing a good and useful service.
So I don’t really think of derailment money as supporting beeminder money.
I guess that’s irrational.
(Possibly relevant: I’ve paid more in premium membership than I have in derailments.)
I’ve shared my feelings at length on the lackluster premium features several times in other places, so I’ll just comment on this from the “ensure Beeminder continues existing” angle.
As a rule I am usually happy to contribute nominally to services I like. I use Patreon.com, I used to pay HabitRPG $5 a month, etc. But Beeminder is a different beast with the pledge system.
I would be willing to pay $5 a month if that money accrued into an account which offset future derails. That account could provide up to, say, 50% of the derail funds so if I owe a $30 derail I could draw $15 from my monthly fee account. In this way, I pay a bit less for derails but 1) they can never be free and 2) the next-level pledge will still be a scary deterrent.
Using such a system, I know that if I never derail I’ll pay the $5 per month but if I derail enough my total spending will approximately equal the total derails since the monthly fee is entirely consumed in the limit.
I’ve thought about this before, and there’s something about the premium features that causes me to pause whenever I consider signing up. I find Beeminder useful; it’s definitely categorized in my mind as a service I am willing to pay for. The way I use it I like to maintain long buffers, and not have too many tasks, so it is unlikely I will pay much for derailments.
Part of the problem is that the premium plans miss the magic price points I have, and seem kind of uninspired. $5 a month is about the limit of what I will give freely to a site to support it, but the cheapest plan comes in at $8. The premium feature I would most like is the ability to automatically trim extra safety buffer, but checking in on goals regularly is something I want to do anyway and 16$ a month is a lot to save a few minutes of clicking to adjust the slope of my goals.
Another piece is that I don’t really understand the business model. It’s not clear if the idea is to have the income coming from derailments or subscriptions. It feels to me like it’s the former, and that I am not a typical user. If I am a more typical user and the site needs people like me signing up at the $16 a month rate to be viable, well then I am dubious about the long term prospects.
I don’t want this to come off as too negative. I like the site and want it to succeed, but whenever I look at the premium support structure I turn away. I’d love to see more premium features that add reporting options, or otherwise make it easier to track and manage goals without derailing. Or the ability to pay a one time payment and unlock the current features.
Alright I’ll bite. Here’s a quote from a post I made last year:
The types of Premium features seem to fall into one of three categories:
A) Features you could get for free by entering fake data.
B) Features that work around quirks or bugs in the website.
C) Non-technical features.
By A) I mean things like configurable retroratchet, trimming safety buffer, and free short-circuiting. Entering fake data feels like a cardinal sin. Beeminder’s non-premium business model is entirely predicated on the hope that some people care about Quantified Self enough to not lie about their data to avoid derailing. I’m one of those people, but these features being dangled in my face feel like Beeminder is tempting me into cheating. It feels backwards from how it ought to be.
By B) I mean things like changing goal URLs. I have a lot of goals whose URLs I would change because I realized too late that the New Goal page drops all the text after the first space. I’m not going to pay $16/month to correct that. If Take a Break or Retroratchet became Premium (per your July 10 post) I would categorize them here, because they make the Road Dial less clunky. (I would also have never joined Beeminder in the first place had those been premium features at the time, but that’s an aside.)
By C) I mean things like fitness tips or the real-time support. These actually might be a cool deal, but they’re bundled in with all the A) and B) type stuff.
agree that the premium plans deserve a bit more love than they’ve gotten, so people are actually willing to sign up for them!! the bulk of revenue does come from derailments, so that’s probably a large part of the lack of attention/development.
[this][1] post i basically agree with in its entirety. i think i would do 2 plan options:
i’m not sure if anyone is beeing beekept at the time, but it’s a separate thing entirely anyway.
love @drtall’s “bank” idea for derailment fees as well, especially with the 50% cap.
[1]: https://groups.google.com/d/msg/akratics/QnzWuaM441M/4u_Cc34COocJ