When I first started working with traffic, I thought there was no difference between mobile and desktop — the main thing was that people clicked. But then I realized that mobile gives excellent conversions on some offers, and almost zero on others. I started digging and, thanks to article https://roiads.co/blog/desktop-or-mobile-traffic/ , I realized that everything depends on the target audience, product, and even time of day. For example, in dating, mobile traffic works much better, but in B2B topics, desktop often wins. There was a time when I simply wasted my budget without taking this factor into account.