Most of the browsing today happens through phone and will continue to rise. If you look around you will hardly find people owning personal laptops. The trend is to either buy a tab or suffice with smart phone.
Many companies block e-commerce sites during work hours. People dont like to open laptops over week end.
So browsing happens mostly on mobiles and you would want to be there when someone is looking out.
What if you have a mobile App and not a website?
It so happens that people first visit the mobile site to check if the listings are interesting and then graduate on to App.