Why Unicode for Coptic?
It is not intended to describe the Unicode concept here. There are several links on the web describing its advantages, specially when it comes to multi language support. If you are not familiar with Unicode, try as a start following links:
Well if you are just happy with entering and printing a text, then you probably don't need Unicode. But if you like to achieve more, like doing text proofing (spell checking), write multi language texts, do word sorting or transforming between upper and lower case and writing portable (i.e. application and platform independent) multi language text, then you should seriously consider switching to Unicode.
Since most of the existing Coptic fonts simply encodes the ASCII range into Coptic characters, your computer keeps "thinking", that you are writing a Latin language. If you have a pure text document, which contains more than one language - let us say for example English and Coptic: if you choose one of those fonts for Coptic to display it, then the English portion of text will be also displayed in Coptic letters. But if you write your text in one of the Unicode formats (for example UTF-8) with a Unicode font, then you can choose any font that defines both languages and both languages will be displayed correctly, no matter which text editor you use, no matter under which operating system you work, no matter if you use "more" or "cat" or whatsoever to display the text.