If you're full busted, cup size D and up, a firm band supports the bust. According to the Brastop video in the link bra straps support 20% of the bust while the band gives most of the support. A well fitting bra actually improves posture because the back is not supporting the weight of the bust, the bra is. I can often tell if a band is too loose because I can feel it twist on me when I move.

When I look at old pictures of me I'm appalled at how poor my posture was due to ill fitting bras. Recent pictures show an entirely different person.