Roku (NASDAQ: ROKU) ended 2017 with over 19 million active accounts using its streaming players and Roku smart TVs. In the company's fourth-quarter letter to shareholders, management said it would be a top three distributor, after AT&T (NYSE: T) and Comcast (NASDAQ: CMCSA), if it were considered a traditional multichannel video programming distributor (MVPD) like them.

Of course, Roku isn't an MVPD. Its platform is often a supplement to a traditional cable or satellite subscription. And while it's another means of distribution for content creators, it's not the primary mode of distribution for premium content.

Nonetheless, there are a couple of big trends supporting the validity of Roku's comparison of itself to cable companies.

Image source: Roku

Time spent watching "TV"

Americans spent an average of about 4.5 hours per day watching television in the second quarter last year. That's down about 12 minutes from the year before, according to data from Nielsen.

A big part of that decline is from continued cord-cutting among consumers, but even active television watchers are spending less time watching TV. TV viewers watched just under six hours per day on average in the second quarter last year. That's down from 6 hours and 22 minutes in 2015.

Meanwhile, the average time spent per user on Roku's platform is climbing. Users spent an average of about 2.6 hours per day streaming on Roku during the fourth quarter.

That's still well short of the time spent on TV, but that number is climbing even as Roku adds millions of new active accounts. Average time spent by Roku users was less than 2.5 hours per day in the fourth quarter last year, and it added nearly 6 million new users over the past four quarters.

As time spent continues to shift from linear television to streaming devices like Roku players and smart TVs, the comparison between Roku and cable becomes much more valuable for thinking of Roku as a means for content creators to distribute their work.

The shift in ad budgets

As users spend less time watching television, it's no surprise that advertisers are looking for different ways to spend their ad budgets. Ad budgets are quickly shifting their balance from television to digital, but television continues to hold on to big branded advertising budgets because of its ability to aggregate large audiences. Streaming video and other forms of digital advertising rarely offer the same kind of audiences that a big live television event can.

But Roku is building technology that offers the best of both worlds -- the targeting of digital with the reach and familiarity of television -- and it may be one of the best replacements for traditional television ad dollars. As such, the scale of its audience becomes an important selling point in its pitch to advertisers.