If there is any economic benefit in presenting a current edition’s contents online, why would there be any less benefit in also presenting the cumulative contents?  If nothing else, this can be a way to continue to draw traffic, and the advertising revenues that follow it, from material that would otherwise have reached the end of its economic usefulness.

Online articles can also be an exceptionally good way for magazines to introduce themselves to new subscribers.   This simply requires that the articles can be found via some form of common, shared content search service, rather than requiring users to go to each individual magazine’s home page and start rummaging around from there.  

When a user finds an individual article he likes, he will tend to check out the magazine as a whole.  If he is favorably impressed, he will be immediately transformed into a good sales prospect—for a magazine he would otherwise never have opened.