To start with the basics, consistency makes a website easier to use, to learn and to remember. Every successful website have a great consistency across all it’s web pages, it’s every easy to browse and to find what you want because every user is able to use and understand it. It is often said that consistency provides a balance between accessibility and usability in designing websites.
On the other hand, every web site should be accessible on all the devices with internet connection (that means smartphones, tables, iPads – not only personal computers and laptops). So, besides the standard rules that have applied until just a few years ago (about the browser compatibility), there are new rules a web designer must apply. Their ment is to get a consistent design and to help users to find the website accessible and usable.