Maybe it's a matter of having resources to move to other types of work where necessarily. After all, if your developer can write documentation and design layouts in Photoshop, you can tell him/her to move to something else when that work needs doing rather than have him sitting around twiddling his/her thumbs while nothing is happening.

I mean, not every company has enough work to keep a developer, technical writer or marketer going full time. Especially not if their product range is limited or like an agency they mostly flutter from random project to random project.

Less optimistically... yeah, it's probably just money and resources. A developer who can design a website layout, write documentation and get it ranking in Google is cheaper than four people doing one job each.