Why do girls? with no boobs even spend money on bras?!

for young girls the technical reason is to get used to wearing a bra because it does feel weird at first. So when they do physically need a bra they'll be used to the feel of it.

But then there are girls who notice their friends are wearing bras and want to fit in.

They're embarassed, and don't want to seem like they're different. Also who wants their nipples to show throw their shirts.

Because nobody wants their nipples to show through their shirt

If they wear padded bras it will give them some shape.

What if they're a grown woman? Are you saying they shouldn't wear bras?

To help promote a nice shape into old age.