A lot of meta usage is overboard if you ask me. Sometimes meta elements are used for Facebook purposes. Other times it is used so the author can stick information in the document. The important uses of meta are to indicate a character set and provide a description for the document.
<meta http-equiv="content-type" content="text/html;charset=utf-8">
<meta http-equiv="content-type" content="text/html;charset=utf-8" />
The charset declaration should be made right at the top of the document after the opening head element. Browsers will use whatever is sent in the HTML header as the character set, ignoring the meta, but this provides information for anyone looking at the HTML.
Another important use of meta is to specify a description for the page which is generally used by search engines in results pages.
<meta name="description" content="This is where you put the description of the page you want to use." >
Also for providing keywords which may people no longer use as search engines ignore them for ranking purposes.
<meta name=”keywords” content="hamburgers, hot dogs, mayo, hot, sun, food, poisoning" >
Another annoying use for meta is to instruct the browser to refresh (or redirect) the page after a period of time, which browsers can be configured to ignore.
Reloads the page after 300 seconds:
<meta http-equiv="refresh" content="300">
Redirects to another website after 5 seconds:
<meta http-equiv="refresh" content="5;url=http://www.sitepoint.com/">
Specifying author information:
<meta name="author" content="cheesedude">
For search engine robots:
<meta name="robots" content="nofollow">
And apparently for defining the viewport size for mobile devices:
I suppose a best practice would be to set the charset and description and not use more than you have to.