Only a very small subset of them: https://bugs.ruby-lang.org/issues/14240

    $/    input record separator (default argument for "gets")
    $\    output record separator ("print" prints it at last)
    $,    default separator for Array#join and print
    $;    default separator for String#split

As for the gain it's because these variables globally change the behavior of important core methods. They really aren't that useful, but can easily cause havoc.