When viewing a website on different devices, we of course expect it to resize to the available device width automatically with no loss of experience; unfortunately, not every site does this quite the right way or successfully!
To understand why this is important, let's assume we operate a desktop version of our site (one in the 1280+ group in this screenshot), and a mobile equivalent from the 418-768 group:
The first stage in making our site responsive is to add the viewport directive; without it, we are likely to end up with a similar effect to this when resizing our sites:
See what I mean? It looks awful—text ...