CSWP is used to query a large data from the Site collections/Sites with the help crawl and indexing in the minimal time.we can customize the display format of the items

Content search webparts or CSWP are used to retrieve and display SharePoint data crawled by search service. It supports filters and refiners to restrict data that gets displayed. It can display data from across site collections. Also the user interface can be customized using display templates which are sheer html and js components.

When visitors browse to a page that contains a CSWP, the Web Part automatically issues a query. The search results are displayed within the CSWP. In most cases, visitors won't even know that search technology is being used to display the content they're viewing. To them, it'll look and feel like any other webpage.By configuring a query in a CSWP, you can define which content visitors will see when they browse to a page that contains a CSWP. By default, the query in the CSWP is configured to show the three most recently changed items on the site.