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Rahul Sethi 11Rahul Sethi 11 

Why do we use custom controllers when we have extensions available to support standard controllers?

sfdcMonkey.comsfdcMonkey.com

Difference between standardcontroller, controller and extensions in Salesforce

 
StandardController
Controller
Extensions
contain the same functionality and logic that are used for standard Salesforce pages.
need finer control for how information is accessed for your page, you can write a custom controller or a controller extension using Apex.
extends the functionality of a standard or custom controller.
Can be used with standard objects and custom objects.
Only one Apex class is used.
Multiple Apex classes separated by comma are used.


and refer below link for deep understadning of Why do we use custom controllers when we have extensions available to support standard controllers :
https://salesforce.stackexchange.com/questions/4041/difference-between-controller-and-extensions 

Hope this will clear your doubts

Let me inform if it helps you

Thanks
Raji MRaji M
Hi Rahul,
Even I am not getting the exact answer to this.
Do you know the answer? If so, can you please explain me!

Thanks,
Raji