Just think of the internet before mobile devices came out. The cost of being online was a bit higher for people because they had to invest in laptops of desktops to connect to the web. Then, the smartphone made an appearance and started to allow people to browse the web from their phones. Being that phones are much cheaper, a larger number of people now browse the web from mobiles.
The problem with this change was that the website content was hard to read because the fonts were too small and you had to do side scrolling just to see the entire site from one side to the other. This forced web designers and developers to come up with