Can't understand apply_filter logic

Never creates self filters before. In my plugin I have variable as array. I want another users can change this array without plugin modification. I'm trying this code in my plugin.php:

<?php
/*
Plugin Name: Test plugin
*/

$arr = [
    'val',
    'val2',
    'val3'
];

$arr = apply_filters( 'my_hook', $arr );
print_r( $arr );

In my function.php I puts this code:

add_filter( 'my_hook', 'modify', 10, 1 );

function modify( $arr ) {
    unset($arr[0]);
    return $arr;
}

I expecting output without first element, but it outputs original array with 3 values;

What's wrong with it?

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