After long months or years of research we want to publish our results and show the community what we have done. Lets take an example: suppose we made some research on Antennas and Propagation, so we definitely want to publish on the IEEE Transactions on Antennas and Propagation. If we want to get published me must pay for it ($110 per printed page) and we must give up all copyrights.
Too bad... but there is no other way. :(
But the process doesn't stop there. We want to publish, we need to read other papers and we need people to read our paper. Is it free? Nooooo! Of course not. If you wanna read a paper on the IEEE Transactions on Antennas and Propagation, you gotta pay for it $30.
Wow! And what IEEE makes with all that money???
IEEE is just an example... many (or maybe all) other Journals make the same.