Economies of scale in sales, tech and business operations. Knowledge sharing, eg. distribution and audience development techniques. Increased leverage with 3rd party vendors, search and social platforms, etc. Plus the usual benefits of scale (cashflow stability etc.)

Partly. I run a network of email newsletters which was inspired by someone suggesting I build a "Weblogs Inc of email newsletters" and the domino effect of branching out into related topics your existing user base is interested in has proven a powerful one.

Another win, which was also true at Weblogs Inc, is the ability to share technology and editorial talent. There are certainly major branding downsides to running 10, say, smaller publications versus 1 bigger one with 10x the traffic, but it's a workable model, if not a glamorous one.

I imagine that it's promoting each other and also centralize the non-editorial parts of running the network. Distributed content staff but centralized sales, technology and administration.