There could be 1000s of journal in the field of biology, which a very broad subject. But certainly there could be limited number of journals for specific subjects e.g. crystallography, microbiology, liver disease, metabolism, diabetes and obesity, cardiology and so on. Even if there are limited number of journals for a particular subject, every journal has different formats, different impact factors, different recognition, different publishers, different editors....
Lets say I have a manuscript specific for cardiology field and I submit it to journal "X". It goes under review and after 1-2 months editor says its rejected. I reformat it, submit it to journal "Y" of the same field, again same thing happens and I go for the next journal. In all this exercise, I waste my 5-6 months and my paper remains still under "REVIEW" in some journal "Z". To avoid this waste of time, if like subject journals come together and form a consortia. Author submit his article to a consortia rather than submitting to one journal. Panel of editors go through the manuscript and decide which journal of the consortia this article suits and then process the article accordingly with author's consent.
Does it sound logical?