It’s not hard to glean why a barely-there, idea-stage startup might want to join an accelerator. The programs are designed to pump your company full of enough resources, networking, and advice to get a beta launch out the door.

The small chunk of cash they offer in exchange for a sliver of equity helps, too.

But why would a profitable, twelve-year-old company join an accelerator? Or how about a TechCrunch Disrupt runner-up with thousands of users? Why would any company with two rounds of institutional funding, or even private equity backing, join an accelerator? Surely they don’t need the money. Their products are shipped, so they don’t need advice there. And why should they give up more equity?

As these questions ran through my head during the FinTech Innovation Lab demo day this morning, I realized something. Startups in the finance industry face a set of challenges so unique that, without help from accelerator, they have no real chance of survival. Finance startups need accelerators because of their mentors -they need someone to teach them to sell to the legacy industry they’re disrupting.

The six startups that FinTech Innovation Lab graduated today seemed successful enough. But the world of finance is so backwards, slow-moving, and risk-averse that even if they managed to get any major institutions to agree to use their products, it’d take them years to actually implement it, facing mountains of red tape and miles of hoops to jump through.

In startup time, a few years is long enough to run out of money.

It’s not even the banks’ fault, necessarily. There is compliance, regulatory oversight, security, and risk to consider. Consumers don’t necessarily want their banks to be terribly innovative. It is the bank’s first responsibility to safely store your money, after all. The elaborate systems of checks and balances are there for a reason.

Still, they’re stifling innovation. “Banks won’t give you a quick yes or no. They’ll give you long maybes until you die,” Yaron Samid, the founder BillGuard told me. “Even if they say yes, it takes 36 months to deploy.” BillGuard came in second at TechCrunch Disrupt’s StartUp Battlefield in 2011. The company also has two rounds of venture backing to the tune of $13 million, and its consumer-facing technology has saved thousands of users upwards of $700,000 in erroneous charges since it launched last year.

All of that is awesome. But in order to truly solve the problem his company was addressing, Samid needed the slow-moving banks to sign on as clients. The other FinTech Innovation Lab startups faced the same selling problem.

And that’s why six capitalized companies with proven concepts and even in some cases “mature” offerings joined the FinTech Innovation Labs accelerator. The program counts high level execs from twelve top financial institutions as mentors, granting the startups unprecedented access to CTOs and CIOs from Goldman Sachs and American Express. That sounds a bit like an advertisement, but I wouldn’t write it if I hadn’t heard each of the participating startups emphasize how crucial the bank relationships were to their success.

“In three months we became a financial technology company. Before that we were a consumer company trying to build an online security system,” Samir told the audience. “Our mentors took us through puberty in three months and we now know how to work with a bank,” he said.

Digital Reasoning, a private equity-backed company founded in 2000, is applying the big data tech it developed for the defense industry to banking. The company had no idea how to sell to banks before its three months with FinTech Innovation Labs, CEO Tim Estes said.

You can certainly debate the value of the fast-proliferating population of accelerators around the country and how much value they add. But more than any other industry, fin-tech needs accelerators.

The question becomes, “Why would the CTOs of Bank of America or CapitalOne want to mentor a group of startups?”

The best answer I got today was simple: They want to stay in front of innovation in their field. They know their organizations aren’t hotbeds of innovation, and they want to change that. They’ve helped the startups in FinTech’s class navigate their organizations and even sped up the sales and implementation process. One person I talked to suggested they want to stay in the good graces of New York’s tech-cheerleading Mayor.

Whatever the reason, the program is helping entrepreneurs take their companies from “decent startup” to “Real Business.” That’s something many accelerators strive to do but simply can’t. It’s also something that shows just how tech-savvy New York is becoming. Our big, stodgy legacy businesses aren’t just watching innovation happen around them — they’re trying to foster it by teaching entrepreneurs how to navigate their industries. The city is brimming with founders who’ve fled jobs in industries like fashion, media, publishing, advertising, or finance to build something that disrupts or improves them. If the leaders of those industries continue to welcome that disruption and innovation with open arms, the startup ecosystem here will become even more of a force to be reckoned with.