why do we write display:flex in css code example

Example 1: css flexbox syntax

Display: flex 
Flex-direction: row | row-reverse | column | column-reverse
align-self: flex-start | flex-end | center | baseline | stretch
justify-content: start |  center | space-between | space-around | space-evenly

Example 2: flexbox css

.container {
  align-content: flex-start | flex-end | center | space-between | space-around | space-evenly | stretch | start | end | baseline | first baseline | last baseline + ... safe | unsafe;
}