Dear Sundar,
MIGO transaction was enhanced in release 4.6 C of SAP, prior to that it was used only for goods receipt for external procurement.
Goods issue was not possible using MIGO in older versions prior to version 4.6C of SAP and transaction MB1A was used. People who have been using SAP for quite a long time, still prefer using MB1A 
There is no functional difference between MB1A and MIGO, instead of remembering multiple transactions for goods issue/receipt cancellation etc, MIGO is a common transaction which can be used.
For complete details about MIGO, please refer below SAP help link,
Enjoy Transaction for Posting Goods Movements (MIGO) - Release notes 46C - SAP Library
Hope this clarifies and helps
Best Regards,
Ameya B.