In case you missed it, Howard Stern again proved his value to Sirius XM (NASDAQ:SIRI) satellite radio with a fascinating "town hall" meeting with songwriting legend Billy Joel. The April 28 show got little attention in advance, but plenty of press afterward, much of it centering around Joel's revelation that he used heroin in the 1970s. Such moments have become a hallmark of Stern's celebrity interviews, and they're a great example of what sets Sirius apart from competitors like Pandora (NYSE:P) and Apple's iRadio.

Those other services are all about content delivery. Sirius, since at least the day Stern came aboard in 2006, has been a content creator in addition to a delivery service.

Adding subscribers through original content

Sirius has grown its revenue in the double-digits for nine consecutive quarters, with the most recent number coming in 11% higher than the year-ago period. It continues to add subscribers, now at 25.8 million, and it expects to cross the 30-million mark within the next few years, CEO Jim Meyer said.

We don't know how many Sirius subscribers tune into Stern. But we know he had an estimated 12 million listeners on terrestrial radio. And we also know that Sirius XM experiences its annual renewal peak in January of each year – not coincidentally, the month Stern started broadcasting over satellite in 2006. What's more important is that we know the content he generates is important to Sirius.

Now 60, Stern and his radio show have evolved from its shock-jock past. Sure, there are still porn-star appearances and outrageous contests. But in an age of over-the-top media, the interview has become the show's true cornerstone. That's likely to attract a different kind of listener, say one who sees Stern on "America's Got Talent," or who reads about the Billy Joel interview.

But Stern is not the be-all, end-all of Sirius's original content. Meyer told analysts recently that he's "never been more excited about what we're doing here at Sirius XM across sports, talk, and music."

Executives touted new programming in news, and a channel dedicated to business and finance led by the Wharton School of Business. Such programming helps Sirius capture some of that long tail of listeners through their interests and occupations.

Is Pandora a worry?

Pandora continues to gain ground in the radio industry. It now boasts 75.3 million active users, up 8% over 2013. And users are listening longer, with total hours up 12% over the year-ago first quarter.

That growth has allowed Pandora to ramp up advertising. It now airs six ads per hour, two more than it previously had, and advertising revenue was up 45% from 2013. It needs that growth to continue. It expects subscription sales to slow, with subscription revenue contributing around 19% to total sales.

That's where Pandora's weakness as a business is apparent. The fact that Pandora remains just a content distributor – no matter how good it is -- is what makes the rise of every new music streamer such a grave concern, whether that's Apple's iRadio, Google's Play Music All Access, or Spotify. Anything that can siphon off listeners with an equally good music platform threatens a major disruption of Pandora's business model. And the big players like Apple And Google aren't relying on their services to make money – only to draw listeners into their ecosystems.

The bottom line

And that's what sets Sirius apart from the competition. Its subscribers are willing to pay for its service, and subscriptions generate 85% of the company's revenue, compared to just 2% that comes in through ads. As long as it can continue to generate its own desirable content on top of the music, sports, and news programming it distributes, its moat becomes deeper and wider, and that much tougher for competitors to cross.

An important question moving forward is how well Sirius is positioning itself to remain a content creator post-Stern. Meyer addressed the need to continue expanding its original programming in the most recent call:

"What I will tell you is that the breadth of content matters a lot ... and that's why we spend what we spend ... and that's why we put the focus on it that we do. ... Creating and broadcasting unrivaled content is the core of our business and vital to attracting new subscribers and keeping existing ones."

As long as it can keep that focus, it looks set to succeed.