Don't they do it because of how ubiquitous Twitter is in many people's lives? The same thing happens on Facebook now, with companies replying to real people who comment on what they've posted. Point is - corporations will find an Internet social communication apparatus; the name doesn't matter.The thing about twitter, is that corporations take it very seriously.
Eh, I bet they do it because it fits well in some corporate checklist and it's easily quantifiable. You don't see that many corporations active on reddit (in an official way, not saying they're not trying to sneak in corporate shills) in comparison.
That might be because Twitter is faceless (it has no identity of its own) and tolerates even worst of humanity's traits being published uncensored. Facebook is similarly faceless: it, like Twitter, is just a platform to post stuff. Reddit, on the other hand, is a subculture with defined traits, one of which is disdain/hatred for corporations. Posting there is as much political (in the sense of being defined by common external interest - Zeitgeist, in this instance) as it is personal.