Why would you use esc_attr() on internal functions?

I see a lot of these in premium themes/plugins.

#1 – Why would you escape this? It’s your own data. For consistency?

function prefix_a() {
    $class_attr = 'a b c';

    // Some more code.

    return '<div class="' . esc_attr( $class_attr ) . '">Content</div>';
}

// Called somewhere.
prefix_a();

#2 – Again, why? The data doesn’t come from the DB.

function prefix_b( $class ) {
    // Some code.

    return '<div class="' . esc_attr( $class ) . '">Content</div>';
}

// Called by a developer from the team.
prefix_b( 'developer adds a class' );

Yes, a child theme developer might call the function above, but he/she is already in control.

#3 – Why? If someone can add filters, it can do a lot more.

function prefix_c() {
    $class_attr = apply_filters( 'prefix_c', 'foo bar' );

    // Some code.

    return '<div class="' . esc_attr( $class_attr ) . '">Content</div>';
}

// Called somewhere.
prefix_c();

I can only think about consistency and to be safe if someone uses untrusted data (excluding the #1 case).