Whether I like it or not, not everyone using my web apps will be running iOS 9 on an iPhone 6S or a Nexus 6P and connecting via super-speedy wifi.
The reality is often anything but that. 3G connections and older hardware is often the norm. Google reports that there are 1.4 billion active Android users. Many of them, will no doubt, be running less-than-top-of-the-line hardware.