Explain extension concept in Hybris e-commerce suite. Why need extension concept in Hybris ?

Hybris e-commerce suite is based on flexible modular concept that means  hybris saying if you want to putting any new functionality then by using extension you can integrate it with hybris.. By using extgen tool you can create own extension in Hybris . Below step follow to create own extension in Hybris.

B-    Open a command prompt.

C-    Navigate to the ${HYBRIS_BIN_DIR}/platform directory.

D-    Run the ${HYBRIS_BIN_DIR}/platform/setantenv.bat file. Do not close the command prompt after this run as the settings are transient and would get lost if the command prompt is closed.

E-    Run ant extgen<ENTER> in ${HYBRIS_BIN_DIR}/platform directory. This internally runs ant in the extgen directory.

 Extgen prompts you to specify:

•  The extension's name
•  The extension's Java package.
•  The extension template to use.  
• For a list of available extension templates compatible with extgen

Need to configure below code in localextensions.xml of config folder

<path autoload="true" dir="${HYBRIS_BIN_DIR}/ext-platform"/>

<path autoload="true" dir="${HYBRIS_BIN_DIR}/ext-cockpit"/>

<path dir="${HYBRIS_BIN_DIR}"/>