When we create a new web app, we get option to select/register new managed account.
I want to know what happens if we create new account, what sort of an account is this?
can someone also provide me with examples of scenarios when we would need to create new managed account?
in a nut shell:
Managed Service Accounts. MSA’s allow you to create an account in Active Directory that is tied to a specific computer. That account has its own complex password and is maintained automatically. This means that an MSA can run services on a computer in a secure and easy to maintain manner, while maintaining the capability to connect to network resources as a specific user
more info here:
the account is used for extra security running services or apps and is handled by the os or by sharepoint!
EDIT
an example of a managed account would be when you install ssrs to sharepoint(intergrated mode) you need a managed account for that application running within sharepoint central admin... when you add ssrs you need a managed account that is part of the domain within central admin and the password change is either never expiers or handled by active directory:
As a best practice Reporting Services 2012 should run under its own domain identity. To configure the Reporting Services 2012 Service Application, an Active Directory account must be created and registered as a managed account in SharePoint Server 2010 or in SharePoint Server 2013
http://support.microsoft.com/kb/2723587
managed accounts are used for less maintanace and ease of use, like creating a new web application... you could create a list of managed accounts and when you create the web application you could easily select from the list rather than typing account name and password! other managed accounts like the example above are specific accounts that are needed for application services to run under sharepoint list of accounts here:
Microsoft SharePoint Server 2010 provides a number of compelling improvements designed especially for the system administrator, of these, commonly overlooked, are Managed Accounts. A Managed Account is effectively an Active Directory user account whose credentials are managed by and contained within SharePoint. In addition to storing the credentials of the object, Microsoft SharePoint Server 2010 can also leverage Active Directory Domain Policies to automatically reset passwords while meeting the requirements established by policy.
from here (Written by Bill Baer himself)
There are more information regarding how you can update, and list, add etc. new managed accounts as well in the link.
