Your development setup is probably a fast laptop, a recent iPhone, office fibre and a set of third-party services that all just work. You test on that setup, and everything looks fine.
Plenty of your users in a new market have something very different. They might have a $150 Android phone with 3 GB of RAM, a prepaid bundle they top up by the gigabyte, a 4G signal that drops out on the bus, and a government that blocks the font service your stylesheet loads first. None of that shows up from your desk. Your page can be perfectly translated, with every date and price right, and still take 20 seconds to appear, or never appear at all.
This chapter covers what people actually have around the world: the phones, the networks and what they cost, the browsers, the services that are blocked or missing, and the local platforms people expect instead. Chapter 22 already covers serving pages from nearby with CDNs and regions. This chapter is about everything between your server and the person reading.