Aeismail have listed several gains as reasons for becoming a journal editor. I think there are also several personal reasons which likely vary between persons. For me, I found that my experience as guest editor for several issues of journals was extremely rewarding. I found I was involved in handling new science at a detailed level unmatched by other venues. I also found myself getting new contacts in the form of authors and reviewers. All along the lines of Aeismail's answer. I strongly recommend trying to get involved in guest editing a single issue if the possibility arises. This might give you an insight into what is involved and how you might like it.
Before continuing, I should perhaps point out that editorships comes in several flavours (with varying names). Editors that are responsible for journals are often called Chief Editors, whereas a staff of other editors may handle reviews and not be involved in making the final decisions for publications and the journal itself. This will vary from journal to journal. The point of saying it is because getting involved as an editor can happen at different levels with differing tasks to perform.
When the opportunity came around for me to shoulder a journal as Editor-in-Chief, I did not need much convincing, but the reasons were more personal than anything else. One could say it is a position of power (to ultimately decide the fate of manuscripts), but I rather think of it in terms of responsibility. I saw it as a challenge to improve the journal and its standing, and to get a chance to implement several ideas I had developed over the years. Assembling a team around me and the second co-chief editor of the journal was also awarding and interesting. So more than anything else, the rewards now come as I can see the Citation Index rising and (hopefully) the standard of the journal improving.
In my case I (really "we" since we are two) get reimbursed for the editorship corresponding to one half day per week. The job is literally 365 days a year, and so far I have not seen this as a problem. The compensation is by no means corresponding to the time I spend on the task but more of a symbolic sum for the responsibility involved. Money is certainly not a reason for me.
To sign up as chief editor likely means signing up for a longer period, at least 3-5 years, to be able to fully embrace the flow of articles and handle all problems that may occur. This is particularly true if you wish to see any results of your work while you are still associated with the journal; it takes time to influence the reputation of a journal in a positive direction. I am not planning to stay forever, so somewhere between 5 and 10 years is likely a maximum. To sign up as review editor (or whatever it may be called) may not require such a long period since the task is more hands on.
So, as you can see I think becoming an editor is not something you just choose, you need to see if you personally get something out of the job that makes it worth your while. For me, just money or personal credit would not be enough, the challenge and sense of contribution associated with leading and improving a journal is.