i've been doing the same thing for years now, since about grade 7 or 8. whenever i'm standing my abs aren't completely clenched, but they're tight. i think i started doing it because it made me look thinner, now i just do it because i've been doing it for so many years. i think it's probably also a healthy thing to do, you're less likely to hurt your back if your core is properly stabilized, so keeping your abs somewhat tight should help prevent back injury. i can't say whether or not it helps me with weight loss or if it's added any abdominal muscle since i've been doing it for so long and back when i started it wasn't very easy to notice a slight change in ab muscle size (i.e. i was overweight, that was probably about my biggest, not my heaviest but definitely my chubbiest).
anyways i'm glad to see it's been working for you, keep it up. AS also helped me lose weight but in a different way. my bike tried to kill me about a month ago (going down a really steep hill the handlebar stem snapped, a car was turning right onto a street right in front of me, i managed to turn onto a sidewalk and coast to a stop without a scratch) so i got a new road bike and i've been obsessed with biking since. I bike everywhere i need to go, which usually adds up to 30mins to an hour a day, and i've been adding in short rides (30mins minimum, usually 45mins-1hr, sometimes 1.5hrs-2hrs) i've lost at least 5lbs since i got my bike and i've been eating a lot more. i also think i've put on some thigh muscle, and my calves have definitely gotten bigger so i may have lost more than 5lbs of fat. the docks have gone in at the rowing club and my exams are done next tuesday, so rowing will start sometime next week (likely thursday) so i should be a lightweight in no time (i've got to lose 10lbs by early july, which will be easy, last year i was at least 5lbs heavier, in worse shape, and i had a car so i wasn't biking all over the place)