Hi Krish,
SAPGUI isn't going to disappear overnight and indeed many older SAP screens built on that technology will be used for many years to come. Personally I can't see SAPGUI being phased out until we move past R/3 to SAP R/4 or whatever it will be called (maybe HCP)....
However the Dynpro technology that is used by the SAPGUI is old UI technology and many newer SAP solutions use Web based technologies such as Web Dynpro and SAPUI5, NWBC can display both SAPGUI (old), Web Dynpro (Newer) and SAPUI5 (Newest) in a single place. That is probably why SAP has advised you to use NWBC, so you can take full advantage of these new UI technologies.
I hope that makes things a bit clearer for you.
Simon