Is it necessary to build a mobile version product website in order to promote mobile app

I was wondering, is it necessary to build a mobile version product website, in order to promote your mobile app? I take Evernote for instance, I do not see they provide a mobile version product website (By surfing the above URL using my Android phone).

I thought is good to have one as potential user of your mobile app, will most probably using mobile device to surf the web. Is there any research or finding on the effectiveness of mobile website, vs desktop website?

It is not necessary to do... but it's a good idea.

I don't have any statistics on this, but I would definitely think making your site responsive would be well worth the effort, particularly if your audience are mobile users.

Coming back to necessary though, there isn't a lot that is necessary (maybe just launching and charging money). Your website isn't a final thing ever, so if you launch without it optimised for mobile and then improve it later, that's fine too.

I would recommend you to focus more on developing and promoting your app. There are many articles on how to boost attention to your app. A website (any version) is a good addition but not a critical. In case if you want to use website as a way to get attention to your app you need to consider that it will take your resources because promoting a website to get high search engine ratings cannot be done in days, more likely in months. You need to work consistently with your site (design, content, ads and so on). If you have folks and funds for your project - use it as an option that will bring fruits but not quickly.

Yes, it's necessary.

You might make your site become responsive in order to achieve that.
Why I say necessary? Because When you build a mobile app, do you believe that it can compatible with any kind of smartphones? As you know that nowadays, there are so many kind of handphones or smartphones, but they have each differences. You build your own mobile app, but maybe you cannot provide the app which is compatible with all types of smartphones.
At least, not in the short time.

If it is so, then it is necessary to build your site responsive.

The answer is both yes and no, with the majority of the time being "no".

First, if you build a desktop website most web developers use "Responsive" themes. These are themes that automatically produce a web AND desktop version from a single source (without needing to produce additional code). This is the most used way to conquer needing to develop for mobile. These themes can be obtained for free if you use wordpress.org, or purchased for a nominal fee (usually 35-75$) through theme websites.

Also, certain startups - like those of Evernote - revolve around Apps found in phone app stores. Evernote receives MOST of it's mobile traffic through the app store. Likewise, the gross majority of those who visit the website do so by desktop. This means the mobile (app) version and desktop versions are how they are because they are built for their primary incoming traffic.

The only time you will see a need to develop for mobile is if your desktop site is specifically designed for a large resolution monitor and you cannot/will not change it.

Hope this clears things up.