In relation to deployment?
Well, a mde is *usually* the result of your final product. ms-access is a
software development tool. You can use ms-access to write a pac-man game, or
even a word processor. When you are done writing the appcation, you turn it
into mde. This mde has all of the extra source code and additional baggage
removed.
From a user point of view, they can't modify a mde. (no more then you can
modify the word, or excel software on your computer). So, a mde is usually
used for final deployment as it is smaller, faster, and cannot be modified
by end users. It is in effect the "last" step you make right before
deploying the appcation to your users.
Since you can't build a mde when you software has any syntax errors, it
also ensures that you are not distribution a application with syntax errors.
Most commercial applications are distributed as a executable. In ms-access,
the mde is that executable. (and, the ms-access problem is thus your runtime
supprot files)