We all know it's crucial to target specific audience when advertising. Not many guys are interested in handbags and not many girls would buy beard oil. But when a niche you're promoting appeals to wider range of people, for example bizopp (business opportunities) and MMO (make-money-online), it may still be worth narrowing down your target audience in advertising.

Exclude Mobile Users
From the SEO point of view, it's important to make sure your site is mobile responsive, as there are more number of internet access from mobile devices than from desktop/laptop. But how much do you actually care about mobile users?
It depends on the niche you're promoting. I have certainly witnessed in my affiliate commission reports that general products (clothing, household, consumables...) are often purchased using a mobile device. People can casually buy stuff and change their mind. They might shop on smartphone at the food court while stuffing their face with pizza and chicken wings. If they don't like what's arrived, they can have it picked up the next day (yes! Amazon does it so good) and receive a refund in full.

Now if you are selling MMO products, you would expect the prospects to be a little more serious than that. What you really don't want is people who only have a mobile access. These products require a commitment. You want like-minded people who are willing to learn about the product and capable to give it a try. Preferably those who have a home office environment where they can focus on their interest. And frankly, people who cannot afford to own a desktop/laptop computer are less likely to purchase your products.
For that reason, when using a PPC (pay-per-click) based advertisement, many marketers do exclude mobile users completely.
Whether you want to invite mobile users to your campaign or not really depends on the type of products. Many internet marketing tools are exclusively made for desktop use. For example sales funnel generators, SEO keyword software, email marketing services, WordPress themes. Whereas there are products specifically made for mobile device such as binary option apps.
T1 Countries Ratio
"Tier 1 (T1) countries" referred by affiliate networks or link tracking services are normally USA, Canada, UK, Australia and New Zealand. English-speaking, reasonably rich countries.

When you use a PPC service with a geographic choice, It is important to choose these 5 countries, plus perhaps Ireland, Singapore, and some European countries where English is widely spoken (The Netherlands, Sweden).
I've done a research over the past year+ to find out the type of people who buy the products I'm promoting. The transactions I was able to verify all turn out to be desktop users from T1 countries. People from the rest of the world are either not interested or not able to afford to buy, quite frankly.
*** Please note my research is at random basis, because affiliate networks don't normally reveal the purchasers' IP address or country of residence in the report. It's my guessing game by matching each date & time in the conversion report against my link tracker.
Web traffic services (such as solo ads) often give you a choice of T1 country ratio. For example;
Option (a): 100 visits for $45, 85% T1 countries or
Option (b): 100 visits for $50, 100% T1 countries.
Option (a) means that if you purchase 100 visits to your opt-in page, you'll only receive 85 visits from the actual prospects. Assuming that people from T2/T3 countries hardly ever buy, the rest of 15 visits will be your 'wasted leads'.
Means in essence, you'll be paying 85 prospect visits for $45 = $0.529 per visit. Whereas with Option (b), you'll be paying 100 prospect visits for $50 = $0.50 per visit. So comparing these 2 options, the option (b) could turn out to be more value for money in this case.
Link Tracker

Link tracking system such as Clickmagick is useful to avoid some useless visits. VPN users, for example. VPN (Virtual Private Network) allows users to mask their location, so the visitors sent to your opt-in page that appear to be from a T1 country may actually not be. A link tracker can detect such visitors in fake location and block them.
Learn How To Target Audience in Advertising
It's important to know your audience and narrow down when advertising in order to receive the maximum effect. You can certainly build your confidence by testing it over time. Learn how to find your audience effectively from here.
How about people who happen to access to your sales funnel on mobile but actually serious entrepreneurs. If you avoid mobile users completely you will lose some valuable prospect also…?
Hi Michael, yes, you can lose serious entrepreneur prospects if you exclude mobile users completely. But it’s better than the risk of welcoming ‘useless’ opt-ins via mobile access like I mentioned in the post. i.e. those who don’t have a desktop access, i.e. will not be able to use the MMO product you’re promoting (depending the product). Advertise a car to everyone including people without drivers licence and children, or advertise it to drivers only (=desktop users). That’s what I meant…I hope it makes sense.
Any more questions please don’t hesitate to ask again. Thanks for your comment.