Newbie question about <?= ?>

Good day, While I've been using php for more than a little while now, I've never understood why the use of the "<?= ...?>" short tag is noted "to be avoided". Or rather, I understand that there's an option to disable it, and that's why it's noted in this way, but I don't understand why it's disabled? What's gained by writing <?php echo some_function(); ?> over <?= some_function(); ?> Thanks in advance. Cheers, Mike