It is a way to fund Paperwork's development. The more money we raise, the more time we can spend working on Paperwork and everything related to it (support, documentation, open-source librairies, etc).
20$ doesn't represent the same thing for 2 different people. Not everybody has the same income (students, unemployed people, etc). We understand that. And we trust you to decide the price you consider honest.
Also, it is a way to measure how much people value our work. You can think of it as a tip.
To measure the value of Paperwork, people must try it first. Once they have, the trial period and the activation code act as a reminder that we need their help to keep improving Paperwork.
Everything remains functional as before, but things get really ugly.
While we believe open-source software is economically viable, we don't believe donations are at Paperwork's scale. It's far too easy to install a program to try it and to forget donating later.
Because we use GNU/Linux everyday. It's a marvelous collection of free and open-source software. It's a joy for us to contribute to this collection.
And because we also use Paperwork for our own personal documents. And having a nice piece of software to handle our documents is a payment in itself. This is why we distribute it for GNU/Linux freely. However, we use it only on GNU/Linux.
Making a version for Microsoft Windows is an extra burden for us. We have no use for it, and it requires a lot of work (porting, testing, support, etc).