There's this whole pseudo-specialty called "biophilic design." I'm no expert and I haven't dug around too much but it basically seems to be the science of "plants are cool."
Green walls started from a basis of "plants are cool" and then there was some science reverse-engineered into it in order to crack money loose. Designers like Ken Yeang cover their buildings in plants 'cuz it's dope and if they can back into some science so much the better. That, I believe, is the justification for the Singapore trees - they look fuckin' cool and Malaysia has been pushing hard into green design so Singapore jumped on, too.
I love the aesthetic. I put up a green wall not because I wanted an air filter but because they look cool. That Boston Fern hanging above the bar is not doing as good a job of making the air breathable as a Smokeeter but a live one is helping more than a silk one.