I found this
<link rel="profile" href="http://gmpg.org/xfn/11"> in
header.php file of WordPress theme. What will happen if I don't use this? What will happen if I use this ?
This profile enables your site to use the FOAF semantic insertions for richer metadata on your site. FOAF goes for Friend Of A Friend and aims to interconnect in "knowledge nodes" all the personal interconnections. Despite it started as a Person to Person oriented RDF vocabulary, it went beyond and included some different larger entities where any PERSON could fit in, like a number of individuals would fit into the same ORGANIZATION. It was quite under the radar until it was de-facto included into the Schema standard promoted by Google, Bing and some other big players in Search.
Theoretically you need to add this profile line in order to correctly use FOAF elements in your site. These elements are indeed recognized and properly named in Google's Structured Data testing tool.
Take a breath and go FOAF Vocabulary Specs
From my personal experience, I can tell you needn't to use the profile line in order to have your metadata in use, but I do use the line as expected.
XFN is a HTML profile which describes the meaning of extra semantic data that can be added to the rel attribute of outbound links.
If you specify relationship information in the link manager for the links you add then this describes the meaning of those attributes to a consumer of your site.
You can read more about XFN here: http://gmpg.org/xfn/
It's a general HTML thing, for more information on the
rel attribute read this
rel="profile" was originally introduced in a draft of XHTML 2.0 (citation needed) as a result of a proposal by Tantek Çelik during an early HTML Working Group meeting (sometime in the early 2000s, working group minutes citation needed) to solve the problem of profile URLs being limited to a single one-off invisible attribute of the element.
This same meaning, that of a link to a metadata profile for the current document, is has been incorporated into XMDP since HTML5 drops the "profile" attribute. See also xmdp-brainstorming, in particular, when microformats may be in use.
My recommendation would be to keep it there