The answer is there within the question itself, appearing pretty obvious.
Well on a serious note, it gives you an opportunity to improve the writer in you. You get a chance to write on a wide variety of topics, there by giving you a flavour of various subjects including arts, science, technology, politics, business, global issues, nature etc.
And in most cases, if you are trying out a freelancer website like odesk.com, elance.com etc. to get writing assignments, chances are most likely you will be writing the article for a global audience, probably for readers who come from a different culture, beliefs, geography compared to yours. Most of these freelance assignments also give importance to the fact that, your content should be original, and as unique and interesting as possible, compared to the rest of articles in the internet on those topics.
This puts your creativity in test, and gives you a challenging time where in you can test your vocabulary as well, and may be fine tune the article in a way which suits the target audience, customized according to geography, culture etc.
There are already a bunch of readers eagerly waiting to read your new article, and isn’t it exciting that you can get the feedback that you are getting on that article from the readers via comments, likes etc, and all this with you getting paid for it.
Charged up to give a test to your writing skills?
You may try one from the following for writing assignments. Wish you all the best!
Do share your experience.