Conventional websites do not display too well on devices with smaller screens.
Earlier people used to design separate websites targeting specific devices like mobiles, tabs etc. The visitor to your site was redirected to the specific mobile friendly site after detecting the device being used by the visitor. This was to ensure that the website was optimally designed and viewable on those specific devices without the need of unnecessary zooming, scrolling, panning etc.
However, with time, the number of devices has increased many folds and is continually on the rise. Therefore, it is no longer possible to design separate websites targeting every specific device. It is costly, time consuming, and difficult to maintain at the same time. Since, this essentially created separate websites, therefore, you ended up having different URLs with duplicate content, which is undesirable and also do not go very well with search engines. Also, the detection process was often flawed sending the visitor to the wrong version of the site.