Most science journals of today are a bad deal for both the scientist who publishes and for the reader. Usually these parties have to pay large amounts of money and also do all the work. Meanwhile the publishers just collect payments and use the material provided by the authors (have in mind that publisher don't even have to format articles since authors are already required to do such a thing).
This situation in addition to being unfair from an ethical point of view it is also highly inconvenient for researchers and developers (specially those of poorer backgrounds). Thus, inciting people to start a free journal in their field is convenient for the people involved and it also provides much needed access to information to those who can't afford it.
The "affordability of information" gap is one of the main obstacles that need to be overcome in order to have the degree of collaboration necessary to solve most of today's problems.