Why is it ideal practice to set width of main container at 1200px

Why is it advised to create a container and set it’s max width to 1200 px and margin to 0 auto,
and put all the child containers inside this main container, what will happen if I don’t provide width to the child containers
eg

css
. container {
max-width:1200px
margin: 0 auto;
}
. navigation {
display: flex
justify-content: space between
}

PS. Ignore any “” and , ;

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