A few years ago I would never have thought I'd be writing in Tagalog or Bisaya. That's starting to change now, as I turn out translations for stories and poems. I owe this shift in perspective to the influence of the veteran members of the Davao Writers Guild.
Why write the same piece in two or more languages? Because it's a challenge. Because there are some things you can express better in one language than the other. Because in that space between languages, there's a thought waiting to come out. I think these are the main reasons.
Oh, sure, there's the matter of audience and heritage, as well, and these are important, too. But compared with the challenge of expressing the inexpressible, they're secondary.