I noticed that
<meta> tags in body only works with
itemprop set, but I have a question. Do search engines read its content?
Yes. This is referred to as "missing and implicit information".
Where the content you wish to mark up with microdata isn't displayed on page in such a way that you have existing markup on which to hang the microdata, you can attach it to
<meta> elements instead.
A good example is VideoObject: a lot of the data you're providing, like duration or the preview image, aren't actually present in the page code.