Enhances curves: If you love clingy tops, but don't want any bulges to show, especially under knitwear, longline bras can mold your shape so that even under the clingiest outfit, you look attractive and shapely.
However, you can also wear a flesh-colored longline or regular bra (flesh will be different for different people, depending on your complexion) beneath it, but it should be full-coverage and not lacy or sheer.
However, while some bustiers are dressy and embellished enough to be worn as club attire, perhaps peeking out from beneath a jacket, longline bras look like bras, made to be worn under your clothing.
Nothing spoils a gorgeous ensemble quicker than an unflattering shape beneath it, so wearing a longline will provide your curves with some control, giving you an enviable hourglass figure.
For women who can't see themselves wearing a regular bra under such a transparent top, consider a longline bra, which will give more coverage, but still provide an air of mystery.