When you want for example enforce the use of the parameter Password if the parameter User is provided. (and vise versa)

Function Do-Something
{
    Param
    (
        [Parameter(Mandatory=$true)]
        [String]$SomeThingToDo,
        [Parameter(ParameterSetName="Credentials", mandatory=$false)]
        [String]$Computername = "LocalHost",
        [Parameter(ParameterSetName="Credentials", mandatory=$true)]
        [String]$User,
        [Parameter(ParameterSetName="Credentials", mandatory=$true)]
        [SecureString]$Password
    )

    #Do something
}

# This will not work he will ask for user and password
Do-Something -SomeThingToDo 'get-help about_Functions_Advanced' -ComputerName

# This will not work he will ask for password
Do-Something -SomeThingToDo 'get-help about_Functions_Advanced' -User