Yahoo CEO Marissa Mayer just announced a complete overhaul to Yahoo's email products.

There's a new version for Android, Windows Phone, iPhone and the Web.

The new design is cleaner – whiter and sparser. There are fewer ads.

In a blog post, Mayer says users "told us loud and clear" that they want "fewer distractions when it comes to email."

"You want to quickly login, communicate, and get on with your day. And we’ve listened."

Rolling out a new version of Yahoo Mail is, by far, the most important thing Mayer has done since she became CEO in July.

There are two reason for this.

The first is that in many ways, all Yahoo is, is a webmail company.

Yahoo has four very important products: Yahoo Sports, Yahoo Finance, the homepage, and Mail. Homepage and Mail are the most important.

Hundreds of millions of people still use Yahoo.com for email.  When they do, Yahoo makes money off them in two ways:

• By showing them ads.
• By giving them links to Yahoo content pages with expensive brand advertising on them.

Yahoo's dependence on Webmail is a problem for the company because usage is declining.

The reason: Adults check their email on their phones, and teens prefer to text. 

It's showing.

Across the entire Internet, Web-based email was up a mere 1% in September 2012 over September 2011, according to ComScore. AOL Mail is down 8%. Yahoo Mail is down 16%. Hotmail is down 18%. Only Gmail is up year-over-year – 16%.

The two questions for Mayer's Webmail redesign are:

• Can it get the kids using email?
• Will it drive usage back to Yahoo.com?

The second reason this rollout is probably the most important thing Mayer has done since she took over Yahoo is Yahoo's desperate situation in mobile.

Right now, Yahoo actually reaches lots of consumers on mobile.

According to ComScore, Yahoo, with 68 million users, has the third largest US mobile audience amongst mobile app-makers, only following Google (100 million) andFacebook (78 million).

But here's the thing: While Yahoo has 70+ apps, the main reason its reach is so big is that the native weather app on iOS is a Yahoo app. 

Yahoo doesn't actually power that app. The weather data is supplied by the Weather Channel, which is pretty good at iOS user-interface design itself. Long term, there is very little reason Apple should keep Yahoo involved.

The truth is, that while Yahoo has lots of reach on mobile, consumers do not actually spend much time with the brand there.