Origin of the tu-tu;
There are two main reasons that ballet dancers wear tutus during performances. One is that it isn't possible for the ballerinas to see their legs; this makes them work harder as the work automatically becomes all the more difficult. The second is due to historical reasons.
Hundreds of years ago ballet dancers skirts were so long that it was rare for ankles to be shown. It was Marie Camargo who, in 1730 made her skirt much shorter. It was after this that shorter skirts became a lot more popular for dancers. It showed people the strength of their legs and how they worked.
To hide the area under the dress from people in the audience who were almost beneath the dancers, a new dress was invented. It was a dress with many layers and frills which joined at the bottom.
The tutu has since evolved into a disc of frills which stick out.
The tutu was made of light material that was extremely flammable. In the 1800's the light source in theatres was gas flame floodlights. Many dancers danced too close and set alight.