Why do we pay to present at conferences? I recently presented at a conference abroad (my first one), and the entry fee was $50 USD. Now, as this was my first conference, the concept of paying to present at a conference is still a foreign one to me. I can't really wrap my head around submitting an abstract, it getting accepted and then paying to present about the work mentioned in it. So my question is fairly straightforward. What is the purpose of a presenter paying for a conference? Isn't it for the greater good not to have barriers for showcasing research? 

Edit: I see this question has generated of a lot of discussion. Let me explain some of my reasoning in choosing the accepted answer. In the region I am from (the Caribbean), due to economic hardships finding the money to pay for conferences deters many young scientists from even thinking about attending. The conferences themselves generate a fairly significant audience from graduate students, faulty members and the general public wishing to understand the scientific concepts presented (all of whom enter for free). I can't help but think how much more productive  the scientific activity in the region would be if  the people wanting to present didn't have to pay. Many of you are saying that $50 USD is nothing, but are neglecting the various exchange rates it takes to obtain said money. At an exchange rate of 121 to 1, $50 USD could buy me food for 2-3 weeks in my home country. I asked this question without any mention of these things because, in the most general sense, paying for conferences seems, to me, to only provide barriers to sharing results and boosting academic (read scientific) activity, and I wanted to have an idea of what others thought about a (somewhat one dimensional) version of this idea.
It costs a lot of money to put conferences on.  Generally, the fees are to help cover this.

In many fields, the majority of people attending are also presenting.  If they didn't charge people to present, they wouldn't be able to generate enough revenue from attendees to make it work.

Note that $50 is very cheap for an academic conference--in my experience, most cost hundreds of dollars. Even then, in many cases the conference fees don't cover the whole cost of the conference, and the difference has to be made up with sponsorship or from other sources.