Journals need publication of articles. The publisher gets articles from the authors but not all of the author's publications as research content was validated or judged by the reviewers. Reviewers who are experts in particular areas get the opportunity to review paper content. So publishers need three things article, author, and reviewer. Big publication houses have hundreds of journals hence publication in such journals needs one more hierarchy that is the editorial board. This board has some relevant field experts who are acting to select or reject the paper. So good researchers have a high chance to become editorial board members of the journal that matches the reviewer profile. Some of the journals Call for Editorial Board Members on the following grounds:
So gathering such board member is to regulate the journal activities smoothly. People who are looking to become permanent members of the journal review community should take some responsibility of the journal and improve the reach of journal without compromising the research quality.
Apart from these responsibilities Call for Editor Board member needs some new subscriptions from the new member. So its not an easy or relaxed designation to maintain for young scholars. Although some experienced professors can maintain this work with the help of previous contacts with universities and research labs. Researchers should not be part of three or more journal as more journals means different task. This is generally free work, journal do not pay for this work except recognition or certificate. But the same thing work in an academic career, some time in the publication of research article as well.