|Why Open Source?|
Simply put, when you buy a proprietary application you limit yourself by one and one only option provided by your web design company. To be fair this may be a perfect solution for you if the company you’re working with accurately represents all your requirements and you are happy to work with it indefinitely. Should the circumstances change though you may find yourself in an awkward situation of being stuck with something you don’t want anymore. And since you won't have the source code, chances are you'd need to start from scratch if you switch to another developer.
By choosing an Open Source solution you free yourself from being locked with one web developer; you get full access to the source code and have opportunity to change and amend your website or application or take it to someone else if you feel the need to do so. You can also modify the application and sell it provided you comply with the original open source license - for example the General Public License, in which case it remains with the code.
To learn more about this topic, here are some useful links: