There are so many job-boards because there are so many open jobs. Though the inching up unemployment rate adds to the number of jobs but that is not the only factor. One of the most important factor is the increasing rate of job-hopping.
While EJ addresses two important issues of connecting Japanese employers with foreigner talent in a market where candidates are less than the open positions, and also the increasing demand of Japanese speaking bilinguals worldwide, why I like it more because of the educational content it provides to the candidates - and all free of cost.