It has become very popular for women to wear t-shirts and tube tops that don't cover the shoulders and this has created a need for bras that don't show while wearing such clothes. This breeded the
strapless bra which is a for of bra that only has a narrow band that goes around the back of the wearer and has no straps to go over the shoulders. The bra can not be seen while wearing the tube tops since everything is covered by the clothes. Also they are usually made completely seamless so you can not see the bumpyness through the t-shirts. Having a lot of different choices for the shirts you are wearing also makes it important to have some different styles of bras to wear to prevent embarassing situations where your bra color is exposed to the bypassers.