Reliable means that the system will work well when you need it - every time. It means that the system will work in a consistant manner so that you can get familiar with it very quickly.

This is important because most people want to use their computer to do things as opposed to continously spending time fixing intermittant error messages and changing things in order to keep your computer working.

It's easy for a computer to eat up time & resources tracking down strange issues. Before you know it the work day is done - but the work you set out to do is not.