Hi,
Query builder is built on top of JCR Query API. When we use query builder, we deal with a map of predicates instead of queries. The Out of the box predicates work in most situations. The limit and offset features work like a charm for pagination situations. Grouping let's you write complex queries in a very readable manner. One more nice feature is the faceted search, your search results can be split by tags. Query builder returns resources and not the nodes, so you don't have to handle to deal with the checked repository exceptions that come along with JCR API ( https://cqdump.wordpress.com/2012/11/06/cq5-coding-patterns-sling-vs-jcr-part-1/ ).
Since it is exposed as a REST servlet, it is often used in building interfaces like custom dashboards and the familiarity helps.
For more details, please refer following articles:
cq5 - CQ QueryBuilder API.. why? - Stack Overflow
http://aempodcast.com/2015/aem-resources/aem-queries-xpath-jcr-sql2-query-builder-syntaxes/#.W9MjTWgzbIU
Exact difference between Jcr-xql2, xpath query and query builder api
I hope this information helps!
Regards,
Lavanya Malyala