Sometimes, you want to search a text field by the length of its content. FieldtypeText doesn't provide this functionality (yet). It's possible to add the necessary SQL to a query by hooking into PageFinder::getQuery like @bernhard pointed out here, but that's of course not really straight forward. There's also an open feature request for FieldtypeText in processwire-requests.
FieldtypeTextWithLength is an extension of FieldtypeText, so you can switch your field's type between the two without loosing any information.
After switching your field to "Text with .length Selector", you can search by the (character, not bytes!) length of the content:
$pagesWithShortHeadlines = $pages->find('headline.length<10');
You will also find a new "Length" subfield when using Pages -> Find in the PW backend: