What are wrapper classes used for in salesforce?

Hello Manpreet

What Is Wrapper Class & How To Use It In Salesforce
A wrapper or container class is a class, data structure, or an abstract data type whose instances are a collections of other objects.It is a custom object defined by Salesforce developer where he defines the properties of the wrapper class. Within Apex & Visualforce this can be extremely helpful to achieve many business scenarios within the Salesforce CRM software.

Use of Wrapper Class within Salesforce:

You can use wrapper class to display records from multiple records within a single table based around the business needs.
For more information you can refer to this link:https://success.salesforce.com/answers?id=90630000000hSoLAAU

Hope this may help: