I am using openSUSE Leap 42.1 Desktop: KDE plasma 5.4.3 . So during login i see two sessions 1. Plasma 5 and 2. KDE workspace. I logged into both sessions and both seem to be the same. So whats the difference between the two and what is the need to have the two separately ?

knightsu wrote:So whats the difference between the two

There is no difference, except for the name.

and what is the need to have the two separately ?

Compatibility with earlier versions.
If somebody chose to login to "KDE Plasma Workspace" previously, (s)he would only get a fallback (twm/IceWM) session after the upgrade otherwise.

Btw, this is highly openSUSE specific, KDE does not ship a "KDE Plasma Workspace" entry with Plasma5.