what does/did tweetdeck do that made it better than the twitter app for android?
Practically everything.
swiping phone to the left/right for columns, the simple yellow (unread) progress bar along right, and just overall efficiency due to it being dead simple. say i have ten things i'm following - first screen is my overall feed, swipe to the right and see all tweets by @DadBoner, swipe one more to the right and get all tweets by others directed @LifeofFitz, swipe to the right again and it might be the #thfc feed, then @TCUCoachP, so on.
fire up tweetdeck, see that my progress bar is 3/4 yellow, crap, got a bunch of crap to read. hmm, wonder if coach patterson has tweeted out any super sweet nature photos? swipe to the right four times. progress bar is slightly yellow, looks like i've got something new to read! ok, done with that, lol at the waterfall. anything new from dadboner? swipe left three times, nothing new. guess joseph hasn't killed him yet. meh, don't really feel like reading all the other new stuff on the main feed i've missed, i'll swipe back to the left one more time and "straight to the top" that bitch. fresh start.
there's probably a lot more advanced stuff that could be done, but that was my basic usage pattern.