Salesforce Sales Cloud is primarily a sales tool while Pardot is a marketing automation platform. Both are designed to increase sales and maximize efficiency for companies with complex sales cycles.
Integrating the two systems lets you synchronize information from both sources, so when you update your CRM records, it automatically updates your marketing automation system (and vice versa). Integration allows sales and marketing to be on the same page in regards to prospect activities, therefore increasing the efficiency of the lead management process. It smooths out the sales funnel, which in turn prevents leads from falling out of the sales process.
Infographic Resource: http://blog.comelite-cloud.com/blog/integrate-pardot-with-salesforce