I don't think there is ever a need for a girl or woman, to wear a padded bra, it is really a matter of choice.
Some girls who wish that they were bigger will wear them to enhance the size of their breasts. Others may wear one as a way to disguise bigger, or erect nipples that can be embarrassing in a thin bra.
Padded bras also give more shape. This is great for younger girls who may be a little too 'pointy' or for older ladies who start to sag (sounds wonderful LOL).
For girls with bigger breasts, padded bras help keep them firmly in place, whereas with a thin bra they can 'jiggle' around.
Under a t-shirt, a padded bra that is also plain (not lacy) gives much better lines and doesn't draw attention. The lace on some bras tends to be very bumpy under some tighter t-shirts.